Re: [RFC PATCH 23/23] x86/process: Reset hardware history in context switch

Peter Zijlstra
2022-10-06T08:36:06Z

On Mon, Oct 03, 2022 at 04:07:58PM -0700, Ricardo Neri wrote:
> On Tue, Sep 27, 2022 at 02:52:24PM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 09, 2022 at 04:12:05PM -0700, Ricardo Neri wrote:
> >
> > > +void reset_hardware_history(void)
> > > +{
> > > + if (!static_cpu_has(X86_FEATURE_HRESET))
> > > + return;
>
> If I used cpu_feature_enabled(X86_FEATURE_ITD) along with the CONFIG_HFI_
> THERMAL and its corresponding DISABLE_MASK bit the code below would be
> compiled out.

Nobody cares about compiled out -- distro's must enable all this. So
what counts is the code size, and the alternative is smaller.

> > > +
> > > + asm volatile("mov %0, %%eax;" __ASM_HRESET "\n" : :
> > > + "r" (hardware_history_features) : "%rax");
> > > +}
> >
> > asm_inline volatile (ALTERNATIVE("", __ASM_HRESET, X86_FEATURE_HRESET)
> > : : "a" (hardware_history_features) : "memory");
>
> Do you still prefer have implemented as an ALTERNATIVE?

Yes, look at the generated code.