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

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Message-ID: <YWbrR1BqI1CxneN/@zn.tnic>
Date:   Wed, 13 Oct 2021 16:20:55 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Willy Tarreau <w@....eu>
Cc:     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>, Michael Matz <matz@...e.de>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list

On Wed, Oct 13, 2021 at 04:07:23PM +0200, Willy Tarreau wrote:
> Yes I agree with the "potentially" here. If it can potentially be (i.e.
> the kernel is allowed by contract to later change the way it's currently
> done) then we have to save them even if it means lower code efficiency.
>
> If, however, the kernel performs such savings on purpose because it is
> willing to observe a stricter saving than the AMD64 ABI, we can follow
> it but only once it's written down somewhere that it is by contract and
> will not change.

Right, and Micha noted that such a change to the document can be done.
And we're basically doing that registers restoring anyway, in POP_REGS.

I'm not the least bit convinced it is worth enforcing that stricter
register saving, though.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette