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    SubjectRe: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry
    On Sat, Oct 15, 2016 at 03:47:45AM -0400, Paolo Bonzini wrote:
    >
    > > > 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

    I'm beginning to think that if a value is always (maybe except for init
    where we don't much care about the code size anyway) accessed through
    *_ONCE macros, we should just mark it volatile and be done with it. The
    code will look cleaner, and there will be less space for errors
    like forgetting *_ONCE macros.

    Would such code (where all accesses are done through
    READ_ONCE/WRITE_ONCE otherwise) be an exception to
    volatile-considered-harmful.txt rules?

    Cc Paul and Jonathan (for volatile-considered-harmful.txt).


    --
    MST

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