Messages in this thread |  | | Date | Sat, 15 Oct 2016 03:47:45 -0400 (EDT) | From | Paolo Bonzini <> | Subject | Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry |
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> > 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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