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SubjectRe: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
On Wed, Oct 13, 2021 at 10:01 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 :-)

--
Ammar Faizi

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