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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

>> 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.

>>  }
>>  
>>  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?

Jan