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

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Message-ID: <20211013163023.GD8557@1wt.eu>
Date:   Wed, 13 Oct 2021 18:30:23 +0200
From:   Willy Tarreau <w@....eu>
To:     Michael Matz <matz@...e.de>
Cc:     Borislav Petkov <bp@...en8.de>,
        Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>,
        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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list

Hello Michael,

On Wed, Oct 13, 2021 at 04:24:28PM +0000, Michael Matz wrote:
(...)
> In short:  Ammars initial claim:
> 
> > 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.
> 
> is accurate and I will clarify the psABI to make that explicit.

Many thanks for this very detailed explanation! Ammar, I'll take your
patch.

Thanks all,
Willy