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

[Dave Hansen]

On 6/22/20 7:52 AM, Liang, Kan wrote:
>>> --- a/arch/x86/kernel/fpu/xstate.c
>>> +++ b/arch/x86/kernel/fpu/xstate.c
>>> @@ -58,6 +58,7 @@ static short xsave_cpuid_features[] __initdata = {
>>> ÂÂ * XSAVE buffer, both supervisor and user xstates.
>>> ÂÂ */
>>> Â 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()
> ÂÂÂ copy_fpregs_to_fpstate()
> ÂÂÂÂÂÂÂ copy_xregs_to_kernel()
> 
> I think we have to export the xfeatures_mask_all.

I'm wondering if we should just take these copy_*regs_to_*() functions
and uninline them.  Yeah, they are basically wrapping one instruction,
but it might literally be the most heavyweight instruction in the whole ISA.

Or, maybe just make an out-of-line version for KVM to call?