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SubjectRe: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry

> > On Oct 14, 2016, at 11:56 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>>
> >>> for (i = 0; i <= 7; i++) {
> >>> - pir_val = xchg(&pir[i], 0);
> >>> - if (pir_val)
> >>> + pir_val = READ_ONCE(pir[i]);
> >>
> >> Out of curiosity, do you really need this READ_ONCE?
> >
> > The answer can only be "depends on the compiler's whims". :)
> > If you think of READ_ONCE as a C11 relaxed atomic load, then yes.
>
> Hm.. So the idea is to make the code "race-free” in the sense
> that every concurrent memory access is done using READ_ONCE/WRITE_ONCE?
>
> If that is the case, I think there are many other cases that need to be
> changed, for example apic->irr_pending and vcpu->arch.pv.pv_unhalted.

There is no documentation for this in the kernel tree unfortunately.
But yes, I think we should do that. Using READ_ONCE/WRITE_ONCE around
memory barriers is a start.

Paolo

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