[musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Nicholas Piggin]

Excerpts from Rich Felker's message of April 16, 2020 8:55 am:
> On Thu, Apr 16, 2020 at 07:45:09AM +1000, Nicholas Piggin wrote:
>> I would like to enable Linux support for the powerpc 'scv' instruction,
>> as a faster system call instruction.
>> 
>> This requires two things to be defined: Firstly a way to advertise to 
>> userspace that kernel supports scv, and a way to allocate and advertise
>> support for individual scv vectors. Secondly, a calling convention ABI
>> for this new instruction.
>> 
>> Thanks to those who commented last time, since then I have removed my
>> answered questions and unpopular alternatives but you can find them
>> here
>> 
>> https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-January/203545.html
>> 
>> Let me try one more with a wider cc list, and then we'll get something
>> merged. Any questions or counter-opinions are welcome.
>> 
>> System Call Vectored (scv) ABI
>> ==============================
>> 
>> The scv instruction is introduced with POWER9 / ISA3, it comes with an
>> rfscv counter-part. The benefit of these instructions is performance
>> (trading slower SRR0/1 with faster LR/CTR registers, and entering the
>> kernel with MSR[EE] and MSR[RI] left enabled, which can reduce MSR 
>> updates. The scv instruction has 128 interrupt entry points (not enough 
>> to cover the Linux system call space).
>> 
>> The proposal is to assign scv numbers very conservatively and allocate 
>> them as individual HWCAP features as we add support for more. The zero 
>> vector ('scv 0') will be used for normal system calls, equivalent to 'sc'.
>> 
>> Advertisement
>> 
>> Linux has not enabled FSCR[SCV] yet, so the instruction will cause a
>> SIGILL in current environments. Linux has defined a HWCAP2 bit 
>> PPC_FEATURE2_SCV for SCV support, but does not set it.
>> 
>> When scv instruction support and the scv 0 vector for system calls are 
>> added, PPC_FEATURE2_SCV will indicate support for these. Other vectors 
>> should not be used without future HWCAP bits indicating support, which is
>> how we will allocate them. (Should unallocated ones generate SIGILL, or
>> return -ENOSYS in r3?)
>> 
>> Calling convention
>> 
>> The proposal is for scv 0 to provide the standard Linux system call ABI 
>> with the following differences from sc convention[1]:
>> 
>> - LR is to be volatile across scv calls. This is necessary because the 
>>   scv instruction clobbers LR. From previous discussion, this should be 
>>   possible to deal with in GCC clobbers and CFI.
>> 
>> - CR1 and CR5-CR7 are volatile. This matches the C ABI and would allow the
>>   kernel system call exit to avoid restoring the CR register (although 
>>   we probably still would anyway to avoid information leak).
>> 
>> - Error handling: I think the consensus has been to move to using negative
>>   return value in r3 rather than CR0[SO]=1 to indicate error, which matches
>>   most other architectures and is closer to a function call.
>> 
>> The number of scratch registers (r9-r12) at kernel entry seems 
>> sufficient that we don't have any costly spilling, patch is here[2].  
>> 
>> [1] https://github.com/torvalds/linux/blob/master/Documentation/powerpc/syscall64-abi.rst
>> [2] https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-February/204840.html
> 
> My preference would be that it work just like the i386 AT_SYSINFO
> where you just replace "int $128" with "call *%%gs:16" and the kernel
> provides a stub in the vdso that performs either scv or the old
> mechanism with the same calling convention. Then if the kernel doesn't
> provide it (because the kernel is too old) libc would have to provide
> its own stub that uses the legacy method and matches the calling
> convention of the one the kernel is expected to provide.

I'm not sure if that's necessary. That's done on x86-32 because they
select different sequences to use based on the CPU running and if the host
kernel is 32 or 64 bit. Sure they could in theory have a bunch of HWCAP
bits and select the right sequence in libc as well I suppose.

> Note that any libc that actually makes use of the new functionality is
> not going to be able to make clobbers conditional on support for it;
> branching around different clobbers is going to defeat any gains vs
> always just treating anything clobbered by either method as clobbered.

Well it would have to test HWCAP and patch in or branch to two 
completely different sequences including register save/restores yes.
You could have the same asm and matching clobbers to put the sequence
inline and then you could patch the one sc/scv instruction I suppose.

A bit of logic to select between them doesn't defeat gains though,
it's about 90 cycle improvement which is a handful of branch mispredicts 
so it really is an improvement. Eventually userspace will stop 
supporting the old variant too.

> Likewise, it's not useful to have different error return mechanisms
> because the caller just has to branch to support both (or the
> kernel-provided stub just has to emulate one for it; that could work
> if you really want to change the bad existing convention).
> 
> Thoughts?

The existing convention has to change somewhat because of the clobbers,
so I thought we could change the error return at the same time. I'm
open to not changing it and using CR0[SO], but others liked the idea.
Pro: it matches sc and vsyscall. Con: it's different from other common
archs. Performnce-wise it would really be a wash -- cost of conditional
branch is not the cmp but the mispredict.

Thanks,
Nick