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

From: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
To: Willy Tarreau <w@1wt.eu>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
Date: Wed, 13 Oct 2021 10:34:39 +0700	[thread overview]
Message-ID: <CAGzmLMXhez2Vvw8+iuBiEe-XRc+vNn8jDww9=aWe=mNk1rHgzA@mail.gmail.com> (raw)
In-Reply-To: <CAGzmLMVaKdBSQrpQFsqEw=Vs240vZANa+1-OLC-nNcKDDevnFQ@mail.gmail.com>

On Wed, Oct 13, 2021 at 10:32 AM Ammar Faizi
<ammar.faizi@students.amikom.ac.id> wrote:
>
> On Wed, Oct 13, 2021 at 10:01 AM Willy Tarreau <w@1wt.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.