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Date: Mon, 20 Apr 2020 14:31:58 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
To: Rich Felker <dalias@libc.org>
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Excerpts from Rich Felker's message of April 20, 2020 2:09 pm:
> On Mon, Apr 20, 2020 at 12:32:21PM +1000, Nicholas Piggin wrote:
>> Excerpts from Rich Felker's message of April 20, 2020 11:34 am:
>> > On Mon, Apr 20, 2020 at 11:10:25AM +1000, Nicholas Piggin wrote:
>> >> Excerpts from Rich Felker's message of April 17, 2020 4:31 am:
>> >> > Note that because lr is clobbered we need at least once normally
>> >> > call-clobbered register that's not syscall clobbered to save lr in.
>> >> > Otherwise stack frame setup is required to spill it.
>> >>=20
>> >> The kernel would like to use r9-r12 for itself. We could do with fewe=
r=20
>> >> registers, but we have some delay establishing the stack (depends on =
a
>> >> load which depends on a mfspr), and entry code tends to be quite stor=
e
>> >> heavy whereas on the caller side you have r1 set up (modulo stack=20
>> >> updates), and the system call is a long delay during which time the=20
>> >> store queue has significant time to drain.
>> >>=20
>> >> My feeling is it would be better for kernel to have these scratch=20
>> >> registers.
>> >=20
>> > If your new kernel syscall mechanism requires the caller to make a
>> > whole stack frame it otherwise doesn't need and spill registers to it,
>> > it becomes a lot less attractive. Some of those 90 cycles saved are
>> > immediately lost on the userspace side, plus you either waste icache
>> > at the call point or require the syscall to go through a
>> > userspace-side helper function that performs the spill and restore.
>>=20
>> You would be surprised how few cycles that takes on a high end CPU. Some=
=20
>> might be a couple of %. I am one for counting cycles mind you, I'm not=20
>> being flippant about it. If we can come up with something faster I'd be=20
>> up for it.
>=20
> If the cycle count is trivial then just do it on the kernel side.

The cycle count for user is, because you have r1 ready. Kernel does not=20
have its stack ready, it has to mfspr rX ; ld rY,N(rX); to get stack to=20
save into.

Which is also wasted work for a userspace.

Now that I think about it, no stack frame is even required! lr is saved=20
into the caller's stack when its clobbered with an asm, just as when=20
it's used for a function call.

>> > The right way to do this is to have the kernel preserve enough
>> > registers that userspace can avoid having any spills. It doesn't have
>> > to preserve everything, probably just enough to save lr. (BTW are
>>=20
>> Again, the problem is the kernel doesn't have its dependencies=20
>> immediately ready to spill, and spilling (may be) more costly=20
>> immediately after the call because we're doing a lot of stores.
>>=20
>> I could try measure this. Unfortunately our pipeline simulator tool=20
>> doesn't model system calls properly so it's hard to see what's happening=
=20
>> across the user/kernel horizon, I might check if that can be improved
>> or I can hack it by putting some isync in there or something.
>=20
> I think it's unlikely to make any real difference to the total number
> of cycles spent which side it happens on, but putting it on the kernel
> side makes it easier to avoid wasting size/icache at each syscall
> site.
>=20
>> > syscall arg registers still preserved? If not, this is a major cost on
>> > the userspace side, since any call point that has to loop-and-retry
>> > (e.g. futex) now needs to make its own place to store the original
>> > values.)
>>=20
>> Powerpc system calls never did. We could have scv preserve them, but=20
>> you'd still need to restore r3. We could make an ABI which does not
>> clobber r3 but puts the return value in r9, say. I'd like to see what
>> the user side code looks like to take advantage of such a thing though.
>=20
> Oh wow, I hadn't realized that, but indeed the code we have now is
> allowing for the kernel to clobber them all. So at least this isn't
> getting any worse I guess. I think it was a very poor choice of
> behavior though and a disadvantage vs what other archs do (some of
> them preserve all registers; others preserve only normally call-saved
> ones plus the syscall arg ones and possibly a few other specials).

Well, we could change it. Does the generated code improve significantly
we take those clobbers away?

Thanks,
Nick