Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

[Thomas Gleixner]

On Tue, 18 Dec 2018, Li, Aubrey wrote:

> On 2018/12/18 22:14, Thomas Gleixner wrote:
> > On Tue, 18 Dec 2018, Aubrey Li wrote:
> >> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> >> index a38bf5a1e37a..8778ac172255 100644
> >> --- a/arch/x86/include/asm/fpu/internal.h
> >> +++ b/arch/x86/include/asm/fpu/internal.h
> >> @@ -411,6 +411,13 @@ static inline int copy_fpregs_to_fpstate(struct fpu *fpu)
> >>  {
> >>  	if (likely(use_xsave())) {
> >>  		copy_xregs_to_kernel(&fpu->state.xsave);
> >> +
> >> +		/*
> >> +		 * AVX512 state is tracked here because its use is
> >> +		 * known to slow the max clock speed of the core.
> >> +		 */
> >> +		if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
> >> +			fpu->avx512_timestamp = jiffies_64;
> > 
> > Even if unlikely this is incorrect when running a 32 bit kernel because
> > there jiffies_64 cannot be atomically loaded vs. a concurrent update. See
> > the comment in include/linux/jiffies.h right above the jiffies_64
> > declaration.
> > 
> Yeah, I noticed this, because this is under use_xsave() condition, also need
> valid AVX512 state, so a 32 bit kernel won't enter this branch.

What exactly prevents a 32bit kernel from having the AVX512 feature bit
set? And if it cannot be set on 32bit, then why are you compiling that code
in at all?

Thanks,

	tglx