Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

[Uros Bizjak]

On Thu, Oct 12, 2023 at 5:19 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
>
>
> > On Oct 12, 2023, at 12:54 AM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > !! External Email
> >
> > On Wed, 11 Oct 2023 at 14:33, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >>
> >> Reading the above, it looks to me that we don't want to play games
> >> with "const aliased" versions of current_task [1], as proposed by
> >> Nadav in his patch series.
> >
> > Well, maybe I'd like it if I saw what the effect of it was, but that
> > patch mentions "sync_mm_rss()" which doesn't actually exist
> > (SPLIT_RSS_COUNTING is never defined, the split version is gone and
> > hasn't existed since commit f1a7941243c1 "mm: convert mm's rss stats
> > into percpu_counter")
>
> So I added a new version of the current aliasing (well, actually pcpu_hot
> in the new version) on top of Uros’s patches, and the effect can be seen
> in many functions. I don’t want to bother with many examples so here is
> a common and simple one:
>
> Currently syscall_exit_work() that starts with:
>
>    0xffffffff8111e120 <+0>: push   %rbp
>    0xffffffff8111e121 <+1>: mov    %rdi,%rbp
>    0xffffffff8111e124 <+4>: push   %rbx
>    0xffffffff8111e125 <+5>: mov    %rsi,%rbx
>    0xffffffff8111e128 <+8>: and    $0x20,%esi
>    0xffffffff8111e12b <+11>: je     0xffffffff8111e143 <syscall_exit_work+35>
>    0xffffffff8111e12d <+13>: mov    %gs:0x2ac80,%rax
>    0xffffffff8111e136 <+22>: cmpb   $0x0,0x800(%rax)
>    0xffffffff8111e13d <+29>: jne    0xffffffff8111e22a <syscall_exit_work+266>
>    0xffffffff8111e143 <+35>: mov    %gs:0x2ac80,%rax
>    0xffffffff8111e14c <+44>: cmpq   $0x0,0x7c8(%rax)
>
> Using the const-alias changes the beginning of syscall_exit_work to:
>
>    0xffffffff8111cb80 <+0>: push   %r12
>    0xffffffff8111cb82 <+2>: mov    %gs:0x7ef0e0f6(%rip),%r12        # 0x2ac80 <pcpu_hot>
>    0xffffffff8111cb8a <+10>: push   %rbp
>    0xffffffff8111cb8b <+11>: mov    %rdi,%rbp
>    0xffffffff8111cb8e <+14>: push   %rbx
>    0xffffffff8111cb8f <+15>: mov    %rsi,%rbx
>    0xffffffff8111cb92 <+18>: and    $0x20,%esi
>    0xffffffff8111cb95 <+21>: je     0xffffffff8111cba6 <syscall_exit_work+38>
>    0xffffffff8111cb97 <+23>: cmpb   $0x0,0x800(%r12)
>    0xffffffff8111cba0 <+32>: jne    0xffffffff8111cc7a <syscall_exit_work+250>
>    0xffffffff8111cba6 <+38>: cmpq   $0x0,0x7c8(%r12)
>
> So we both see RIP-relative addressing is being used (hence the instruction is
> one byte shorter) and the reload going away.

Just a quick remark here:

For some reason existing percpu_stable_op asm uses %P operand
modifier. This will drop all syntax-specific prefixes and issue the
bare constant. It will also remove the (%rip) suffix. What we want
here is a generic %a modifier (See 6.47.2.8 Generic Operand Modifiers
[1]) that will substitute a memory reference, with the actual operand
treated as the address. In combination with "p" constraint will DTRT
and will emit symbol with the (%rip) suffix when available, also when
-fpie is in effect.

[1] https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc.pdf

Uros.