Re: [PATCH 5/5] x86_64: add 64-bit syscall entry point

[Sergey Bugaev]

On Tue, Feb 28, 2023 at 4:26 PM Luca Dariz <luca@orpolo.org> wrote:
> >> +    /* check if we need to place some arguments on the stack */
> >> +_syscall64_args_stack:
> >> +    mov     EXT(mach_trap_table)(%rax),%r10 /* get number of arguments */
> >> +    subq    $6,%r10                 /* the first 6 args are already in 
> >> place */
> >> +    jl      _syscall64_call         /* skip argument copy if >6 args */
> >
> > jle?
>
> Right, I didn't test a 6-args syscall.
>
> >> +
> >> +    movq    R_UESP(%rbx),%r11       /* get user stack pointer */
> >> +    addq    $8,%r11                 /* Skip user return address */
> >> +
> >> +    mov     $USER_DS,%r12           /* use user data segment for accesses 
> >> */
> >> +    mov     %r12,%fs
> >> +
> >> +    lea     (%r11,%r10,8),%r11      /* point past last argument */

Do I understand it right that for the most interesting syscall (which
takes 7 args!), I *am* supposed to pass the 7th arg on the stack (in
mem[rsp + 8]) -- unlike on Linux?

Or in other words: do I understand it right that the ABI here is:

- syscall number in rax
- arguments are passed just as per x86_64 calling convention, except
the 4th arg is in r10 and not rcx
- return value is in rax
- rcx and r11 are additionally clobbered -- or not?
- nothing else is clobbered, in particular not rflags (or is the
"reserved" half of rflags clobbered?) and not the registers that
contain args

Sergey