From: Peter Zijlstra <peterz@infradead.org>
To: Brian Gerst <brgerst@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
LKML <linux-kernel@vger.kernel.org>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
"H . Peter Anvin" <hpa@zytor.com>,
Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH v2 09/10] x86/percpu: Clean up percpu_stable_op()
Date: Thu, 9 Jul 2020 12:25:50 +0200 [thread overview]
Message-ID: <20200709102550.GN597537@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAMzpN2hHSeN9Z3BO_Uo1-yCfh9YdEbkK0wb9E3D8rq6onQDZdg@mail.gmail.com>
On Tue, Jun 02, 2020 at 10:19:51AM -0400, Brian Gerst wrote:
> On Mon, Jun 1, 2020 at 4:43 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Sat, May 30, 2020 at 3:11 PM Brian Gerst <brgerst@gmail.com> wrote:
> > >
> > > Use __pcpu_size_call_return() to simplify this_cpu_read_stable().
> >
> > Clever! As in this_cpu_read() in include/linux/percpu-defs.h. Could
> > be its own patch before this, but it's fine.
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> >
> > > Also remove __bad_percpu_size() which is now unused.
> > >
> > > Signed-off-by: Brian Gerst <brgerst@gmail.com>
> > > ---
> > > arch/x86/include/asm/percpu.h | 41 ++++++++++-------------------------
> > > 1 file changed, 12 insertions(+), 29 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > > index 7efc0b5c4ff0..cf2b9c2a241e 100644
> > > --- a/arch/x86/include/asm/percpu.h
> > > +++ b/arch/x86/include/asm/percpu.h
> > > @@ -85,7 +85,6 @@
> > >
> > > /* For arch-specific code, we can use direct single-insn ops (they
> > > * don't give an lvalue though). */
> > > -extern void __bad_percpu_size(void);
> > >
> > > #define __pcpu_type_1 u8
> > > #define __pcpu_type_2 u16
> > > @@ -167,33 +166,13 @@ do { \
> > > (typeof(_var))(unsigned long) pfo_val__; \
> > > })
> > >
> > > -#define percpu_stable_op(op, var) \
> > > -({ \
> > > - typeof(var) pfo_ret__; \
> > > - switch (sizeof(var)) { \
> > > - case 1: \
> > > - asm(op "b "__percpu_arg(P1)",%0" \
> >
> > What does the `P` do here?
> > https://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints
> > says can be machine dependent integral literal in a certain range.
> > https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
> > doesn't document `P` for x86 though...
>
> https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Extended-Asm.html#x86-Operand-Modifiers
>
> Removing the 'P' modifier results in this:
> movq %gs:$current_task, %rdx #, pfo_val__
>
> This is because the 'p' constraint treats a memory address as a
> constant. I tried replacing it with __this_cpu_read(), which since
> commit 0b9ccc0a should have similar non-volatile semantics. But the
> compiler still reloaded it on every use, so I left the asm template
> as-is for now until that can be resolved.
Right, I can into that same issue a while back and gave up staring at
compiler innards. __this_cpu_read() *should* allow re-loads, and it does
in places (we've had bugs because of it), but this_cpu_read_stable() is
somehow far more 'effective'.
It would be good if someone can update the comment with that thing, to
explain matters better.
next prev parent reply other threads:[~2020-07-09 10:26 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-30 22:11 [PATCH v2 00/10] x86: Clean up percpu operations Brian Gerst
2020-05-30 22:11 ` [PATCH v2 01/10] x86/percpu: Introduce size abstraction macros Brian Gerst
2020-06-01 20:48 ` Nick Desaulniers
2020-05-30 22:11 ` [PATCH v2 02/10] x86/percpu: Clean up percpu_to_op() Brian Gerst
2020-07-09 10:30 ` Peter Zijlstra
2020-07-10 4:38 ` Brian Gerst
2020-07-10 8:53 ` Peter Zijlstra
2020-07-10 16:56 ` Nick Desaulniers
2020-05-30 22:11 ` [PATCH v2 03/10] x86/percpu: Clean up percpu_from_op() Brian Gerst
2020-05-30 22:11 ` [PATCH v2 04/10] x86/percpu: Clean up percpu_add_op() Brian Gerst
2020-06-01 20:18 ` Nick Desaulniers
2020-05-30 22:11 ` [PATCH v2 05/10] x86/percpu: Remove "e" constraint from XADD Brian Gerst
2020-06-01 18:50 ` Nick Desaulniers
2020-05-30 22:11 ` [PATCH v2 06/10] x86/percpu: Clean up percpu_add_return_op() Brian Gerst
2020-06-01 19:47 ` Nick Desaulniers
2020-05-30 22:11 ` [PATCH v2 07/10] x86/percpu: Clean up percpu_xchg_op() Brian Gerst
2020-05-30 22:11 ` [PATCH v2 08/10] x86/percpu: Clean up percpu_cmpxchg_op() Brian Gerst
2020-05-30 22:11 ` [PATCH v2 09/10] x86/percpu: Clean up percpu_stable_op() Brian Gerst
2020-06-01 20:43 ` Nick Desaulniers
2020-06-02 14:19 ` Brian Gerst
2020-07-09 10:25 ` Peter Zijlstra [this message]
2020-05-30 22:11 ` [PATCH v2 10/10] x86/percpu: Remove unused PER_CPU() macro Brian Gerst
2020-06-01 20:26 ` Nick Desaulniers
2020-06-01 21:00 ` [PATCH v2 00/10] x86: Clean up percpu operations Nick Desaulniers
2020-07-08 19:36 ` Nick Desaulniers
2020-07-09 20:02 ` Nick Desaulniers
2020-07-13 22:24 ` Nick Desaulniers
2020-07-13 22:40 ` Linus Torvalds
2020-07-13 22:58 ` Nick Desaulniers
2020-07-14 0:31 ` Nick Desaulniers
2020-07-14 1:29 ` Linus Torvalds
2020-07-09 10:30 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200709102550.GN597537@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=bp@alien8.de \
--cc=brgerst@gmail.com \
--cc=hpa@zytor.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@kernel.org \
--cc=mingo@kernel.org \
--cc=ndesaulniers@google.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).