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SubjectRe: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state
On 2018-09-17 10:37:20 [+0200], Paolo Bonzini wrote:
> > . This is (again)
> > untested since I have no box with this PKRU feature. This only available
> > on Intel's Xeon Scalable, right?
>
> It is available on QEMU too (the slower JIT thing without KVM, i.e. use
> /usr/bin/qemu-system-x86_64 instead of /usr/bin/qemu-kvm or /usr/bin/kvm).

okay. I managed to get the kernel to report this flag as available now.

> > - if (preload)
> > - __fpregs_load_activate(new_fpu, cpu);
> > + if (!preload)
> > + return;
> > +
> > + __fpregs_load_activate(new_fpu, cpu);
> > +
> > +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> > + /* Protection keys need to be in place right at context switch time. */
> > + if (boot_cpu_has(X86_FEATURE_OSPKE)) {
> > + if (new_fpu->pkru != __read_pkru())
> > + __write_pkru(new_fpu->pkru);
> > + }
> > +#endif
>
> I think this would be before the "if (preload)"?

I did not find an explicit loading of pkru except this "xrestore"
which happens on "preload". From what I saw, preload was always set
except for kernel threads. Based on that, it looks to me like it can be
skipped if there is no FPU/kernel thread.

> >
> > if (boot_cpu_has(X86_FEATURE_OSPKE))
> > - copy_init_pkru_to_fpregs();
> > + pkru_set_init_value();
> > }
> >
>
> Likewise, move this to fpu__clear and outside "if
> (static_cpu_has(X86_FEATURE_FPU))"?

okay. But if there is no FPU we did not save/restore the pkru value. Is
this supposed to be an improvement?

> Also, a __read_pkru() seems to be missing in switch_fpu_prepare.

But the value is stored during write_pkru(). So the copy that is saved
should be identical to the value that would be read, correct?

>
> Thanks,
>
> Paolo

Sebastian

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