linux-kernel - Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list

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Message-ID: <CAGzmLMVV2i1HYZ06fayUx4=1-TdvnKB=8KK_UAiLqeD4qg_NCw@mail.gmail.com>
Date:   Wed, 13 Oct 2021 10:37:29 +0700
From:   Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>
To:     Willy Tarreau <w@....eu>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list

On Wed, Oct 13, 2021 at 10:34 AM Ammar Faizi
<ammar.faizi@...dents.amikom.ac.id> wrote:
>
> On Wed, Oct 13, 2021 at 10:32 AM Ammar Faizi
> <ammar.faizi@...dents.amikom.ac.id> wrote:
> >
> > On Wed, Oct 13, 2021 at 10:01 AM Willy Tarreau <w@....eu> wrote:
> > >
> > > On Wed, Oct 13, 2021 at 05:23:11AM +0700, Ammar Faizi wrote:
> > > > According to x86-64 ABI about syscall section A.2 AMD64 Linux Kernel
> > > > Conventions, A.2.1 Calling Conventions [1]:
> > > >
> > > > 1) User-level applications use as integer registers for passing the
> > > >    sequence %rdi, %rsi, %rdx, %rcx, %r8 and %r9. The kernel interface
> > > >    uses %rdi, %rsi, %rdx, %r10, %r8 and %r9.
> > > >
> > > > 2) A system-call is done via the syscall instruction. The kernel
> > > >    destroys registers %rcx and %r11.
> > > >
> > > > 3) The number of the syscall has to be passed in register %rax.
> > > >
> > > > 4) System-calls are limited to six arguments, no argument is passed
> > > >    directly on the stack.
> > > >
> > > > 5) Returning from the syscall, register %rax contains the result of
> > > >    the system-call. A value in the range between -4095 and -1
> > > >    indicates an error, it is -errno.
> > > >
> > > > 6) Only values of class INTEGER or class MEMORY are passed to the
> > > >    kernel.
> > > >
> > > > From (2), (5) and (6), we can conclude that Linux x86-64 syscall only
> > > > clobbers rax, rcx and r11 (and "memory").
> > > >
> > > >   - rax for the return value.
> > > >   - rcx to save the return address.
> > > >   - r11 to save the rflags.
> > > >
> > > > Other registers are preserved. Right?
> > >
> > > I totally agree, and this doc is perfectly clear on this. I think it
> > > would be worth updating the comments in calling.h to reference this
> > > document and remind these rules, given that they're not trivial to
> > > figure from the code itself.
> > >
> >
> > Alright, I will wire up patch v2 for this :-)
> >
>
> Not for calling.h, nolibc.h I mean.

If Borislav, or others agree with my reply above, it's fine if one can do that
for calling.h (or maybe better in entry_64.S)?

-- 
Ammar Faizi