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

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Message-ID: <CAGzmLMVaKdBSQrpQFsqEw=Vs240vZANa+1-OLC-nNcKDDevnFQ@mail.gmail.com>
Date:   Wed, 13 Oct 2021 10:32:03 +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 10:01 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 :-)

-- 
Ammar Faizi