Re: [Qemu-devel] [PATCH] linux-user: provide frame information in x86-64 safe_syscall

[Richard Henderson]

On 05/27/2016 09:34 AM, Peter Maydell wrote:
> On 27 May 2016 at 17:21, Richard Henderson <address@hidden> wrote:
> > On 05/27/2016 08:06 AM, Peter Maydell wrote:
> > > @@ -31,6 +32,8 @@ safe_syscall_base:
> > >           * does not list any ABI differences regarding stack alignment.)
> > >           */
> > >          push    %rbp
> > > +        .cfi_def_cfa_offset 16
> > > +        .cfi_offset rbp,-16
> >
> >
> > While this is correct, there are two other directives that make it easier to
> > describe changes without having to compute globally correct constants.  Here
> > they would be:
> >
> >         .cfi_adjust_cfa_offset 8
> >
> > Add 8 to the offset, i.e. decrement the SP by 8.
>
> Presumably .cfi_startproc sets the initial offset to 8?
> (It's not documented that it does so, which is I think partly why
> I preferred to use a directive that definitely set the offset
> to the right thing.)

It is documented to set up the normal no-instructions-executed call frame. 
Which in the case of x86, does have a non-zero offset.

There is a ".cfi_startproc simple" that begins a frame with no opcodes at all.


r~