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Date:   Thu, 12 Oct 2023 18:55:06 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Thu, Oct 12, 2023 at 6:33 PM Uros Bizjak <ubizjak@...il.com> wrote:
>
> On Thu, Oct 12, 2023 at 5:19 PM Nadav Amit <namit@...are.com> wrote:
> >
> >
> > > On Oct 12, 2023, at 12:54 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > >
> > > !! External Email
> > >
> > > On Wed, 11 Oct 2023 at 14:33, Uros Bizjak <ubizjak@...il.com> 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.

An example:

--cut here--
int m;

int foo (void)
{
  asm ("# %0 %P0 %a0" :: "p" (&m));
}
--cut here--

gcc -O2 -S:

       # $m m m(%rip)

gcc -O2 -fpie -S:

       # $m m m(%rip)

gcc -O2 -m32 -S:

       # $m m m

Uros.

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