From: Uros Bizjak <ubizjak@gmail.com>
To: Nadav Amit <namit@vmware.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andy Lutomirski <luto@kernel.org>,
Brian Gerst <brgerst@gmail.com>,
Denys Vlasenko <dvlasenk@redhat.com>,
"H . Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
Date: Thu, 12 Oct 2023 18:55:06 +0200 [thread overview]
Message-ID: <CAFULd4bBeMCBf5_VKWA8Ui=rhQtXf03zFUioq1DeQaYCfP8v=A@mail.gmail.com> (raw)
In-Reply-To: <CAFULd4Zp-eDsxpStBznMHUE3OcHZ97NAZrZEjJW63oEFWtM3OQ@mail.gmail.com>
On Thu, Oct 12, 2023 at 6:33 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Thu, Oct 12, 2023 at 5:19 PM Nadav Amit <namit@vmware.com> wrote:
> >
> >
> > > On Oct 12, 2023, at 12:54 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > >
> > > !! External Email
> > >
> > > On Wed, 11 Oct 2023 at 14:33, Uros Bizjak <ubizjak@gmail.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.
next prev parent reply other threads:[~2023-10-12 16:55 UTC|newest]
Thread overview: 106+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-10 16:42 [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr() Uros Bizjak
2023-10-10 17:32 ` Linus Torvalds
2023-10-10 18:22 ` Uros Bizjak
2023-10-10 18:25 ` Nadav Amit
2023-10-10 18:42 ` Linus Torvalds
2023-10-10 18:37 ` Linus Torvalds
2023-10-10 18:41 ` Uros Bizjak
2023-10-10 18:52 ` Linus Torvalds
2023-10-11 7:27 ` Uros Bizjak
2023-10-11 7:45 ` Uros Bizjak
2023-10-11 19:40 ` Linus Torvalds
2023-10-11 18:42 ` Uros Bizjak
2023-10-11 19:51 ` Linus Torvalds
2023-10-11 19:52 ` Linus Torvalds
2023-10-11 20:00 ` Uros Bizjak
2023-10-11 22:37 ` Ingo Molnar
2023-10-11 23:15 ` H. Peter Anvin
2023-10-12 1:35 ` Josh Poimboeuf
2023-10-12 6:19 ` Ingo Molnar
2023-10-12 16:08 ` Josh Poimboeuf
2023-10-12 17:59 ` Ingo Molnar
2023-10-12 21:30 ` Josh Poimboeuf
2023-10-13 10:52 ` Ingo Molnar
2023-10-11 7:41 ` Nadav Amit
2023-10-11 19:37 ` Linus Torvalds
2023-10-11 21:32 ` Uros Bizjak
2023-10-11 21:54 ` Linus Torvalds
2023-10-12 15:19 ` Nadav Amit
2023-10-12 16:33 ` Uros Bizjak
2023-10-12 16:55 ` Uros Bizjak [this message]
2023-10-12 17:10 ` Linus Torvalds
2023-10-12 17:47 ` Linus Torvalds
2023-10-12 18:01 ` Uros Bizjak
2023-10-13 9:38 ` Uros Bizjak
2023-10-13 11:53 ` Uros Bizjak
2023-10-13 16:38 ` Linus Torvalds
2023-10-12 17:52 ` Uros Bizjak
2023-11-20 9:39 ` Use %a asm operand modifier to obtain %rip-relative addressing Uros Bizjak
2023-10-12 16:56 ` [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr() Linus Torvalds
2023-10-12 17:16 ` Linus Torvalds
2023-10-12 19:32 ` Nadav Amit
2023-10-12 19:40 ` Linus Torvalds
2023-10-16 18:52 ` Uros Bizjak
2023-10-16 19:24 ` Linus Torvalds
2023-10-16 20:35 ` Nadav Amit
2023-10-16 20:59 ` Linus Torvalds
2023-10-16 23:02 ` Linus Torvalds
2023-10-16 23:14 ` Linus Torvalds
2023-10-17 7:23 ` Nadav Amit
2023-10-17 19:00 ` Linus Torvalds
2023-10-17 19:11 ` Uros Bizjak
2023-10-17 21:05 ` Uros Bizjak
2023-10-17 21:53 ` Linus Torvalds
2023-10-17 22:06 ` Nadav Amit
2023-10-17 22:29 ` Nadav Amit
2023-10-18 7:46 ` Uros Bizjak
2023-10-18 9:04 ` Uros Bizjak
2023-10-18 10:54 ` Nadav Amit
2023-10-18 12:14 ` Uros Bizjak
2023-10-18 13:15 ` Uros Bizjak
2023-10-18 14:46 ` Nadav Amit
2023-10-18 15:17 ` Uros Bizjak
2023-10-18 16:03 ` Nadav Amit
2023-10-18 16:26 ` Linus Torvalds
2023-10-18 17:23 ` Uros Bizjak
2023-10-18 18:11 ` Linus Torvalds
2023-10-18 18:08 ` Uros Bizjak
2023-10-18 18:15 ` Linus Torvalds
2023-10-18 18:26 ` Uros Bizjak
2023-10-18 19:33 ` Uros Bizjak
2023-10-18 20:17 ` Nadav Amit
2023-10-18 20:22 ` Linus Torvalds
2023-10-18 20:34 ` Linus Torvalds
2023-10-18 20:51 ` Uros Bizjak
2023-10-18 21:09 ` Uros Bizjak
2023-10-18 21:10 ` Linus Torvalds
2023-10-18 21:40 ` Uros Bizjak
2023-10-18 22:40 ` Linus Torvalds
2023-10-18 23:06 ` Linus Torvalds
2023-10-19 7:04 ` Uros Bizjak
2023-10-19 16:59 ` Linus Torvalds
2023-10-19 17:21 ` Uros Bizjak
2023-10-19 18:06 ` Linus Torvalds
2023-10-19 18:16 ` Uros Bizjak
2023-10-19 18:49 ` Linus Torvalds
2023-10-19 19:07 ` Linus Torvalds
2023-10-20 7:57 ` Uros Bizjak
2023-10-19 21:04 ` Linus Torvalds
2023-10-19 22:39 ` Linus Torvalds
2023-10-20 8:08 ` Uros Bizjak
2023-10-19 8:44 ` Peter Zijlstra
2023-10-19 8:54 ` Peter Zijlstra
2023-10-19 17:04 ` Linus Torvalds
2023-10-19 18:13 ` Peter Zijlstra
2023-10-19 18:22 ` Linus Torvalds
2023-10-19 18:37 ` Uros Bizjak
2023-10-19 9:07 ` Peter Zijlstra
2023-10-19 9:23 ` Uros Bizjak
2023-10-18 20:42 ` Uros Bizjak
2023-10-19 16:32 ` Uros Bizjak
2023-10-19 17:08 ` Linus Torvalds
2023-10-18 18:29 ` Nadav Amit
2023-10-18 16:12 ` Linus Torvalds
2023-10-18 17:07 ` Uros Bizjak
2023-10-18 18:01 ` Linus Torvalds
2023-10-16 21:09 ` Uros Bizjak
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