Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full instruction mask

["Liang, Kan" <kan.liang@linux.intel.com>]

On 6/19/2020 3:31 PM, Peter Zijlstra wrote:
> On Fri, Jun 19, 2020 at 07:04:05AM -0700, kan.liang@linux.intel.com wrote:
> 
>> KVM includes the header file fpu/internal.h. To avoid 'undefined
>> xfeatures_mask_all' compiling issue, xfeatures_mask_all has to be
>> exported.
> 
>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index 587e03f..eb2e44e 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -58,6 +58,7 @@ static short xsave_cpuid_features[] __initdata = {
>>    * XSAVE buffer, both supervisor and user xstates.
>>    */
>>   u64 xfeatures_mask_all __read_mostly;
>> +EXPORT_SYMBOL_GPL(xfeatures_mask_all);
> 
> *groan*...
> 
> AFAICT KVM doesn't actually use any of those functions,

It seems KVM may eventually invoke copy_xregs_to_kernel() as below.

kvm_save_current_fpu()
     copy_fpregs_to_fpstate()
         copy_xregs_to_kernel()

I think we have to export the xfeatures_mask_all.

Thanks,
Kan

> can't we have
> something like BUILD_KVM (like BUILD_VDSO) and exclude those functions
> from the KVM build?
> 
> I so detest exporting random crap because kvm..
>