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

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