Ways of Implementing a System Call(x86.lol)
x86.lol
Ways of Implementing a System Call
https://x86.lol/generic/2019/07/04/kernel-entry.html
57 comments
> Modulate the CPU temperature by running code. The OS decodes the signal. Use message passing that involves OFDM, LDPC, ASN.1, and digital signatures.
I like how you mixed this in with real suggestions.
I like how you mixed this in with real suggestions.
It's an excellent suggestion if the code you're trying to communicate with is not actually the kernel, but rather other code on the machine that the kernel is attempting to prevent you from communicating with. Except for the ASN.1 part. That's never a good idea.
Or code not on the machine. I remember: "PowerHammer: Exfiltrating Data from Air-Gapped Computers Through Power Lines" https://news.ycombinator.com/item?id=16821513
> Except for the ASN.1 part. That's never a good idea.
Don't kink-shame the ITU-T! They're just sadomasochistic, that's all...
Don't kink-shame the ITU-T! They're just sadomasochistic, that's all...
Finally, a perfect application for INTERCAL.
Nothing from that list is sane i386 syscall mechanism :).
(Which given the fact that the list contains undocumented and obscure 0xf1 icebp and not documented 0xcc int3 seems to be the intention)
(Which given the fact that the list contains undocumented and obscure 0xf1 icebp and not documented 0xcc int3 seems to be the intention)
I'd do an integer division by zero. The syscall number would be the dividend, and zero is the divisor.
An elegant, brilliant idea. What could possibly go wrong? ;)
How about floating point division by zero? Then you could use floating point syscall numbers! SWOON
Call it the NaNix kernel.
"To infinity, and beyond!!!"
How about floating point division by zero? Then you could use floating point syscall numbers! SWOON
Call it the NaNix kernel.
"To infinity, and beyond!!!"
> What could possibly go wrong? ;)
It's not a bug, it's a feature. This adds.. ehem.. a new dimension to integer numbers (or even complex numbers, we'll call them supercomplex numbers).
It's not a bug, it's a feature. This adds.. ehem.. a new dimension to integer numbers (or even complex numbers, we'll call them supercomplex numbers).
Brilliant! For example, something like 180.7 could then be understood by the kernel to "userland wants to call 70% open and 30% unlink" in one go!
You could probably build an academic career on "fractional interpolation of parameterized system calls".
Quantum computing supremacy!
Quantum Objects: Maybe that's what "Quajects" in Alexia Massalin’s Synthesis Kernel actually represent!
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.29....
p. 51: Chapter 4, Kernel Structure
All things should be made as simple as possible, but no simpler. -Albert Einstein
4.1 Quajects
Quajects are the building blocks out of which all Synthesis kernel services are composed. The name is derived from the term "object" of Object-Oriented (O-O) systems, which they strongly resemble [32]. The similarity is strong, but the difference is significant. Like objects, quajects encapsulate data and provide a well-defined interface to access it. Unlike objects, quajects use a code-synthesis implementation to achieve high performance, but lack high-level language support and inheritance.
Kernel quajects can be broadly classified into four kinds: thread, memory, I/O, and device. Thread quajects encapsulate the unit of execution, memory quajects the unit of data storage, I/O quajects the unit of data movement, and device quajects the machine's interfaces to the outside world. Each kind of quaject is defined and implemented independently.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.29....
p. 51: Chapter 4, Kernel Structure
All things should be made as simple as possible, but no simpler. -Albert Einstein
4.1 Quajects
Quajects are the building blocks out of which all Synthesis kernel services are composed. The name is derived from the term "object" of Object-Oriented (O-O) systems, which they strongly resemble [32]. The similarity is strong, but the difference is significant. Like objects, quajects encapsulate data and provide a well-defined interface to access it. Unlike objects, quajects use a code-synthesis implementation to achieve high performance, but lack high-level language support and inheritance.
Kernel quajects can be broadly classified into four kinds: thread, memory, I/O, and device. Thread quajects encapsulate the unit of execution, memory quajects the unit of data storage, I/O quajects the unit of data movement, and device quajects the machine's interfaces to the outside world. Each kind of quaject is defined and implemented independently.
That might not work. Integer division by zero is defined to raise #DE, incidentally the very first in the interrupt vector. Floating point division by zero silently produces infinity or NaN without informing the supervisor.
Is PC-AT style "get the keyboard controller to reset the CPU, then have special BIOS code detect situation and jump to your code?" still available in modern systems? Real mode is sortof a supervisor mode.
Of course you would have to make the KBC low level accessible to user mode which may cause issue processing legacy input devices. Plus having to reinitialise protected mode every system call might cause some efficiency issues.
Of course you would have to make the KBC low level accessible to user mode which may cause issue processing legacy input devices. Plus having to reinitialise protected mode every system call might cause some efficiency issues.
IIRC enough of the KBC is still present that this might actually work, another question is whether modern UEFI firmware actually handles the SW-induced reboot.
As for entering and exiting protected mode: that is in fact surprisingly fast operation assuming that all the relevant data structures are still in place. In fact DOS XMS managers (and VxD-based Windows) switch between PM with vm86, true real mode and various unreal-ish (ie. real mode with weird values in descriptor shadow registers) modes quite regularly.
As for entering and exiting protected mode: that is in fact surprisingly fast operation assuming that all the relevant data structures are still in place. In fact DOS XMS managers (and VxD-based Windows) switch between PM with vm86, true real mode and various unreal-ish (ie. real mode with weird values in descriptor shadow registers) modes quite regularly.
actually, Linux uses at least two of these:
for security reasons (ASLR), the vsyscall page is implemented on modern kernels by unmapping the page, trapping the page fault, and emulating the system call.
the recently introduced io_uring interface provides an interface to set up a buffer ring which is asynchronously polled by the kernel for system calls to execute.
for security reasons (ASLR), the vsyscall page is implemented on modern kernels by unmapping the page, trapping the page fault, and emulating the system call.
the recently introduced io_uring interface provides an interface to set up a buffer ring which is asynchronously polled by the kernel for system calls to execute.
oh, and of course if you count the vdso as "making a syscall", then three: page fault, polling, and reverse polling.
Before you look at these benchmarks and concluded system calls are cheap, please be aware the real cost is not 100 cycles, it's 100 cycles plus the cost of clobbering your L1 and often L2 cache, instruction cache, TLB, etc. This is made worse on many current systems due to mitigations for Intel's buggy processors. The real cost is often more than 10,000-30,000 cycles. Which is why for anything very syscall heavy, like a network server handling millions of very small packets per second you get more than an order of magnitude better performance by bypassing the kernel and avoiding syscalls altogether. It's also why threads are "faster" than processes even although they're implemented the same way, no need to flush the TLB on context switch.
Is it possible to use larger chunks of data when working with packets from the kernel?
Any idea on how this would apply to something like clock_gettime(CLOCK_MONOTONIC) ? This seems to have extremely low overhead ...
That's one of the functions provided by the vDSO, so userspace code can call it without a context switch: http://man7.org/linux/man-pages/man7/vdso.7.html
The current trend seems to be to use ring buffers between the kernel and userspace (see: perf, io_uring, this proposal https://lwn.net/Articles/789603/), which reduce this overhead.
At the lowest level this is how it works, so using ring buffers as the abstraction makes tremendous sense. Plus it can be done safely without requiring a context switch on the fast path, with a little care.
Its also easy to naturally batch things. When the load is low the batch size is 1, and then it naturally grows as more work gets queued during processing of the prior work.
I love ring buffers and queues.
Its also easy to naturally batch things. When the load is low the batch size is 1, and then it naturally grows as more work gets queued during processing of the prior work.
I love ring buffers and queues.
Although you are technically correct in that the cost of system calls on most systems is high, your post also implies to a casual reader that the cost is fundamental. This is not true. As demonstrated in the benchmark, the hardware cost of systems calls/trapping is quite low. Even accounting for hardware workarounds (which were not even known about when the common consensus decided system calls were slow), excepting the case of hardware virtualization which has much more expensive mitigations, the fundamental cost is a fraction of the actual cost.
The majority of the cost (~90+%) is actually due to software. In the system call case this is the implementation of the system call. In the context switch case this is usually the cost of the scheduling decision. These operations take material amounts of time and access memory which evicts user caches. It is in speeding up the software component that real gains can be made. This is not to say that system calls could ever be made "free" since function calls/loads/stores will still be ~100x faster, but the cost would probably be ~10% of the current cost if they software component could magically be made instant.
As an example of what I am talking about, there is this talk by Google from 2013 [1] which describes a "fast context switch" system call (benchmark at 18:40). This "context switch" knows what it is switching to which lets it bypass the scheduling decision in normal context switches. As a result it takes ~170 ns (which is pretty close to the hardware cost mentioned in the OP) vs 2000 ns of a normal context switch.
I could go into more detail as to some of the causes of the underlying software costs, but that is a discussion for another time, so I will just leave this off with summarizing my point.
tl;dr: System calls are slow. It is due to the software implementing them, not the hardware.
[1]: https://www.youtube.com/watch?v=KXuZi9aeGTw
The majority of the cost (~90+%) is actually due to software. In the system call case this is the implementation of the system call. In the context switch case this is usually the cost of the scheduling decision. These operations take material amounts of time and access memory which evicts user caches. It is in speeding up the software component that real gains can be made. This is not to say that system calls could ever be made "free" since function calls/loads/stores will still be ~100x faster, but the cost would probably be ~10% of the current cost if they software component could magically be made instant.
As an example of what I am talking about, there is this talk by Google from 2013 [1] which describes a "fast context switch" system call (benchmark at 18:40). This "context switch" knows what it is switching to which lets it bypass the scheduling decision in normal context switches. As a result it takes ~170 ns (which is pretty close to the hardware cost mentioned in the OP) vs 2000 ns of a normal context switch.
I could go into more detail as to some of the causes of the underlying software costs, but that is a discussion for another time, so I will just leave this off with summarizing my point.
tl;dr: System calls are slow. It is due to the software implementing them, not the hardware.
[1]: https://www.youtube.com/watch?v=KXuZi9aeGTw
Would BPF count as "bypassing the kernel" or are you referring to hardware acceleration?
e.g. https://blog.cloudflare.com/cloudflare-architecture-and-how-...
e.g. https://blog.cloudflare.com/cloudflare-architecture-and-how-...
BPF is more like moving your compute into the kernel, but it achieves the same effect - you avoid context switch overhead associated with syscalls. Kernel bypass is typically using ring buffers and a poll-mode network driver (e.g. DPDK, snabbswitch) and skipping the whole kernel network stack.
Fun fact, not only can you use iret to return to user-mode from kernel-mode and kernel-mode from kernel-mode (as footnote 5 mentions), you can also use iret to return from user-mode to user-mode. It is a handy way to restore RIP, RSP, and RFLAGS at the same time. .NET CoreCLR uses it during exception handling on Unix:
https://github.com/dotnet/runtime/blob/4db72366e8e49c30d7aa6...
https://github.com/dotnet/runtime/blob/4db72366e8e49c30d7aa6...
I heard that, especially with spectre, most of the cost of context switching comes from messing up your caches + TLB (https://blog.tsunanet.net/2010/11/how-long-does-it-take-to-m...). There's a graph on Dan Luu's blog about how it takes 14,000 cycles to get back to full performance after a syscall: https://danluu.com/images/new-cpu-features/flexsc_ipc_recove....
I wonder if it would be possible (& worth doing, since it would require cross cutting work) to enable saving and loading cache hit profiles
What do you mean? If you can record the data in the first place you can probably save it. If you mean making some kind of workload or benchmark, couldn't you write some code to do it? It's probably somewhat inefficient because you can't write to it directly but you can get pretty close.
Sounds like something profile-guided optimization might do.
The problem is that on i386 the TLB entries are keyed only by linear address and not by page table that the entry came from and writing into the page table register (even rewriting the same value) is documented as triggering TLB flush.
On typical RISC platforms that directly expose TLB to supervisor TLB entries usually have few bits that can be used to record for which process is the entry valid and thus there is no need to flush whole TLB on each task switch.
(Modern i386 CPUs actually have somewhat similar mechanism, but it is only usable by hypervisors, not by normal operating systems)
On typical RISC platforms that directly expose TLB to supervisor TLB entries usually have few bits that can be used to record for which process is the entry valid and thus there is no need to flush whole TLB on each task switch.
(Modern i386 CPUs actually have somewhat similar mechanism, but it is only usable by hypervisors, not by normal operating systems)
One of the ways that many operating systems (I'm fairly certain that Windows did this at one point) implemented syscalls long ago was to execute an illegal instruction, because this was the fastest way at the time to transition to kernel mode. x86 even has a specific opcode that is guaranteed to never be used aka always be an illegal instruction
OK, I really don't know anything about assembly/OSes, but if the price of going to kernel mode with sysenter/syscall is smaller than a not that pricey(the article says it's less than a single 64-bit integer division), does that mean microkernels can utilize that to improve performance?
What are the shortcomings of the method compared to interrupts?
What are the shortcomings of the method compared to interrupts?
The l4 variants think so. Unfortunately Spectre was sort of a pain and now they have to switch page table roots a bunch.
On microkernels the issue is usually not with syscall performance but with essentially every operation causing a task switch which on i386 causes TLB flush (which in post-Spectre days seems somewhat moot) and in many cases involves interacting with task scheduler.
(QNX uses interesting hack to sidestep this: the message passing interface does not really pass messages, but is simply cross-addressspace call/return that in essence temporarily moves a thread to different process without rescheduling anything)
(QNX uses interesting hack to sidestep this: the message passing interface does not really pass messages, but is simply cross-addressspace call/return that in essence temporarily moves a thread to different process without rescheduling anything)
Saying that the cost of a system call is X cycles is IMHO a bit "short", what's the effect on the various caches? TLB..
Plus nearly all system calls must validate the user inputs, so the minimum cost for a system cost is far from the sysenter cost itself.
Plus nearly all system calls must validate the user inputs, so the minimum cost for a system cost is far from the sysenter cost itself.
L4 is all about removing all of those concerns from at least a variant of IPC, FWIW.
That domain name though!
Another super-efficient way of implementing a system call is described in Alexia Massalin's PhD classic thesis on the Synthesis Kernel:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.29....
>Synthesis: An Efficient Implementation of Fundamental Operating System Services
>This dissertation shows that operating systems can provide fundamental services an order of magnitude more efficiently than traditional implementations. It describes the implementation of a new operating system kernel, Synthesis, that achieves this level of performance.
>The Synthesis kernel combines several new techniques to provide high performance without sacrificing the expressive power or security of the system. The new ideas include:
>- Run-time code synthesis -- a systematic way of creating executable machine code at runtime to optimize frequently-used kernel routines | queues, buffers, context switchers, interrupt handlers, and system call dispatchers | for specific situations, greatly reducing their execution time.
>- Fine-grain scheduling -- a new process-scheduling technique based on the idea of feedback that performs frequent scheduling actions and policy adjustments (at submillisecond intervals) resulting in an adaptive, self-tuning system that can support real-time data streams.
>- Lock-free optimistic synchronization is shown to be a practical, efficient alternative to lock-based synchronization methods for the implementation of multiprocessor operating system kernels.
>- An extensible kernel design that provides for simple expansion to support new kernel services and hardware devices while allowing a tight coupling between the kernel and the applications, blurring the distinction between user and kernel services.
>The result is a significant performance improvement over traditional operating system implementations in addition to providing new services.
Previous discussion on HN:
The Synthesis Kernel (1988) [pdf] (usenix.org)
https://news.ycombinator.com/item?id=15076642
https://www.usenix.org/legacy/publications/compsystems/1988/...
It’s Time for a Modern Synthesis Kernel (regehr.org)
https://news.ycombinator.com/item?id=20337231
https://blog.regehr.org/archives/1676
>Alexia Massalin’s 1992 PhD thesis has long been one of my favorites. It promotes the view that operating systems can be much more efficient than then-current operating systems via runtime code generation, lock-free synchronization, and fine-grained scheduling. In this piece we’ll only look at runtime code generation, which can be cleanly separated from the other aspects of this work.
Valerie Henson's commentary on the Synthesis kernel (2008) (lwn.net)
https://news.ycombinator.com/item?id=10441995
https://lwn.net/Articles/270081/
KHB: Synthesis: An Efficient Implementation of Fundamental Operating Systems Services
>When I was but a wee computer science student at New Mexico Tech, a graduate student in OS handed me an inch-thick print-out and told me that if I was really interested in operating systems, I had to read this. It was something about a completely lock-free operating system optimized using run-time code generation, written from scratch in assembly running on a homemade two-CPU SMP with a two-word compare-and-swap instruction - you know, nothing fancy. The print-out I was holding was Alexia (formerly Henry) Massalin's PhD thesis, Synthesis: An Efficient Implementation of Fundamental Operating Systems Services (html version here). Dutifully, I read the entire 158 pages. At the end, I realized that I understood not a word of it, right up to and including the cartoon of a koala saying "QUA!" at the end. Okay, I exaggerate - lock-free algorithms had been a hobby of mine for the previous few months - but the main point I came away with was that there was a lot of cool stuff in operating systems that I had yet to learn.
>Every year or two after that, I'd pick up my now bedraggled copy of "Synthesis" and reread it, and every time I would understand a little bit more. First came the lock-free algorithms, then the run-time code generation, then quajects. The individual techniques were not always new in and of themselves, but in Synthesis they were developed, elaborated, and implemented throughout a fully functioning UNIX-style operating system. I still don't understand all of Synthesis, but I understand enough now to realize that my grad student friend was right: anyone really interested in operating systems should read this thesis.
Other interesting mentions:
https://news.ycombinator.com/item?id=19598385
>Massalin's Synthesis kernel [10] stands out to me as another example that for me crosses the boundaries and is both outstanding engineering and art in challenging the ideas of how systems could be built in ways that make me look at it just as much because of the beauty of it as because of the practical ideas. (The main thing it brought was the idea of making the kernel adapt to its clients by generating custom code for system calls; to me it is art because it turned the idea of a kernel as something static on its head; and conceptually we're still just scraping the very surface of dynamic code generation in kernels that Massalin's thesis started playing with). There are many works like that, where the specific implementations are irrelevant - nobody uses the Synthesis kernel - but where the ideas are as important as any expressed in more overt or "intentional" art.
https://news.ycombinator.com/item?id=20507155
>ggm: It feels to me like if you can do runtime code call checks, and confirm which actual calls you make, then stripping the bits of libc and associated libraries out, being left with only the strictly required calls, and then by extension the syscalls, and then by extension the kernel elements, is actually possible a lot of the time. So, library -> reduced library -> reduced calls -> reduced syscalls -> reduced kernel state is a sequence or set or something, of applied minimisations which can be done, if you can predict all the call paths in your code and their dependencies.
>NelsonMinar: That sounds a little like the ideas in the Synthesis kernel, the idea of the kernel JIT-compiling itself to optimize your code path.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.29....
>Synthesis: An Efficient Implementation of Fundamental Operating System Services
>This dissertation shows that operating systems can provide fundamental services an order of magnitude more efficiently than traditional implementations. It describes the implementation of a new operating system kernel, Synthesis, that achieves this level of performance.
>The Synthesis kernel combines several new techniques to provide high performance without sacrificing the expressive power or security of the system. The new ideas include:
>- Run-time code synthesis -- a systematic way of creating executable machine code at runtime to optimize frequently-used kernel routines | queues, buffers, context switchers, interrupt handlers, and system call dispatchers | for specific situations, greatly reducing their execution time.
>- Fine-grain scheduling -- a new process-scheduling technique based on the idea of feedback that performs frequent scheduling actions and policy adjustments (at submillisecond intervals) resulting in an adaptive, self-tuning system that can support real-time data streams.
>- Lock-free optimistic synchronization is shown to be a practical, efficient alternative to lock-based synchronization methods for the implementation of multiprocessor operating system kernels.
>- An extensible kernel design that provides for simple expansion to support new kernel services and hardware devices while allowing a tight coupling between the kernel and the applications, blurring the distinction between user and kernel services.
>The result is a significant performance improvement over traditional operating system implementations in addition to providing new services.
Previous discussion on HN:
The Synthesis Kernel (1988) [pdf] (usenix.org)
https://news.ycombinator.com/item?id=15076642
https://www.usenix.org/legacy/publications/compsystems/1988/...
It’s Time for a Modern Synthesis Kernel (regehr.org)
https://news.ycombinator.com/item?id=20337231
https://blog.regehr.org/archives/1676
>Alexia Massalin’s 1992 PhD thesis has long been one of my favorites. It promotes the view that operating systems can be much more efficient than then-current operating systems via runtime code generation, lock-free synchronization, and fine-grained scheduling. In this piece we’ll only look at runtime code generation, which can be cleanly separated from the other aspects of this work.
Valerie Henson's commentary on the Synthesis kernel (2008) (lwn.net)
https://news.ycombinator.com/item?id=10441995
https://lwn.net/Articles/270081/
KHB: Synthesis: An Efficient Implementation of Fundamental Operating Systems Services
>When I was but a wee computer science student at New Mexico Tech, a graduate student in OS handed me an inch-thick print-out and told me that if I was really interested in operating systems, I had to read this. It was something about a completely lock-free operating system optimized using run-time code generation, written from scratch in assembly running on a homemade two-CPU SMP with a two-word compare-and-swap instruction - you know, nothing fancy. The print-out I was holding was Alexia (formerly Henry) Massalin's PhD thesis, Synthesis: An Efficient Implementation of Fundamental Operating Systems Services (html version here). Dutifully, I read the entire 158 pages. At the end, I realized that I understood not a word of it, right up to and including the cartoon of a koala saying "QUA!" at the end. Okay, I exaggerate - lock-free algorithms had been a hobby of mine for the previous few months - but the main point I came away with was that there was a lot of cool stuff in operating systems that I had yet to learn.
>Every year or two after that, I'd pick up my now bedraggled copy of "Synthesis" and reread it, and every time I would understand a little bit more. First came the lock-free algorithms, then the run-time code generation, then quajects. The individual techniques were not always new in and of themselves, but in Synthesis they were developed, elaborated, and implemented throughout a fully functioning UNIX-style operating system. I still don't understand all of Synthesis, but I understand enough now to realize that my grad student friend was right: anyone really interested in operating systems should read this thesis.
Other interesting mentions:
https://news.ycombinator.com/item?id=19598385
>Massalin's Synthesis kernel [10] stands out to me as another example that for me crosses the boundaries and is both outstanding engineering and art in challenging the ideas of how systems could be built in ways that make me look at it just as much because of the beauty of it as because of the practical ideas. (The main thing it brought was the idea of making the kernel adapt to its clients by generating custom code for system calls; to me it is art because it turned the idea of a kernel as something static on its head; and conceptually we're still just scraping the very surface of dynamic code generation in kernels that Massalin's thesis started playing with). There are many works like that, where the specific implementations are irrelevant - nobody uses the Synthesis kernel - but where the ideas are as important as any expressed in more overt or "intentional" art.
https://news.ycombinator.com/item?id=20507155
>ggm: It feels to me like if you can do runtime code call checks, and confirm which actual calls you make, then stripping the bits of libc and associated libraries out, being left with only the strictly required calls, and then by extension the syscalls, and then by extension the kernel elements, is actually possible a lot of the time. So, library -> reduced library -> reduced calls -> reduced syscalls -> reduced kernel state is a sequence or set or something, of applied minimisations which can be done, if you can predict all the call paths in your code and their dependencies.
>NelsonMinar: That sounds a little like the ideas in the Synthesis kernel, the idea of the kernel JIT-compiling itself to optimize your code path.
[deleted]
> Although Call Gates are the somewhat official way of implementing system calls in the absence of the more modern alternatives discussed below, I’m aware of no use except by malware.
I think 386BSD and early FreeBSD used call gates for syscalls.
I think 386BSD and early FreeBSD used call gates for syscalls.
Was reading this article while waiting for my kernel to build.
bwrawp giving my LFS nightmares, and I'm not super keen on user_namespace, but suid isn't working either, FML
bwrawp giving my LFS nightmares, and I'm not super keen on user_namespace, but suid isn't working either, FML
I wonder if mainstream CPUs will ever get back to being truly programmable with software instructions, instead of running off and speculating on what you might do next.
Most people don't want a much slower processor. As soon as you have a pipeline the concepts of "now" and "next" become smeared out in time.
Would also save some watts I guess
The 0xf1 byte, an "icebp" or "int1" instruction
In 32-bit code, use an FPU stack underflow
In 64-bit mode, access a non-canonical address
In 32-bit code, use the "into" instruction
Write data using a segment selector that goes beyond the LDT limit
With the alignment checking enabled, do a misaligned memory operation
Of course, we can also do system calls without causing exceptions. Examples:
Write the system call number to a well-known memory location that the OS will poll
Modulate the CPU temperature by running code. The OS decodes the signal. Use message passing that involves OFDM, LDPC, ASN.1, and digital signatures.