Re: [PATCH] x86/FPU: make vcpu_reset_fpu() build again with old gcc

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

On Tue Dec 10, 2024 at 2:34 PM GMT, Jan Beulich wrote:
> On 10.12.2024 15:25, Alejandro Vallejo wrote:
> > On Mon Dec 9, 2024 at 3:13 PM GMT, Jan Beulich wrote:
> >> Fields of anonymous structs/unions may not be part of an initializer for
> >> rather old gcc.
> > 
> > Can you add the specific version for tracking purposes?
>
> It's all the same as before, and I really didn't want to waste time on
> once again figuring out which exact version it was that the behavior
> changed to the better.

Just checked on Godbolt. 4.7.1 works and 4.6.4 doesn't. Adding that data point
to the commit message really helps when navigating git-blame, even if it's not
as precise as it could be. Particularly if one wants to understand exactly
which quirk of which version of which compiler is being dealt with.

>
> >> Fixes: 49a068471d77 ("x86/fpu: Rework fpu_setup_fpu() uses to split it in 
> >> two")
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>
> >> --- a/xen/arch/x86/i387.c
> >> +++ b/xen/arch/x86/i387.c
> >> @@ -306,13 +306,13 @@ void vcpu_reset_fpu(struct vcpu *v)
> >>  {
> >>      v->fpu_initialised = false;
> >>      *v->arch.xsave_area = (struct xsave_struct) {
> >> -        .fpu_sse = {
> >> -            .mxcsr = MXCSR_DEFAULT,
> >> -            .fcw = FCW_RESET,
> >> -            .ftw = FXSAVE_FTW_RESET,
> >> -        },
> >>          .xsave_hdr.xstate_bv = X86_XCR0_X87,
> >>      };
> >> +
> >> +    /* Old gcc doesn't permit these to be part of the initializer. */
> >> +    v->arch.xsave_area->fpu_sse.mxcsr = MXCSR_DEFAULT;
> >> +    v->arch.xsave_area->fpu_sse.fcw = FCW_RESET;
> >> +    v->arch.xsave_area->fpu_sse.ftw = FXSAVE_FTW_RESET;
> > 
> > That's not quite the same though. A more apt equivalence would be to memset 
> > the
> > area to zero ahead of the assignments. Otherwise rubble will be left behind.
>
> No. I didn't delete the initializer. All fields not mentioned there will
> be default-initialized.

Right. I misread the diff and thought you had nuked the initializer. That's
indeed all fine. Which means...

>
> >>  }
> >>  
> >>  void vcpu_setup_fpu(struct vcpu *v, const void *data)
> > 
> > Out of context and not triggering the GCC bug, but vcpu_setup_fpu() should
> > probably share the same initialization style as vcpu_reset_fpu(), imo.
>
> Why?

... there's indeed no reason to touch that.

>
> Jan

With the commit message adjusted with the offending GCC version (i.e: <4.7.1):

  Acked-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>

Cheers,
Alejandro