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    SubjectRe: [v3 23/26] KVM: Update Posted-Interrupts Descriptor when vCPU is preempted


    On 12/12/2014 16:14, Feng Wu wrote:
    > + if (irq_remapping_cap(IRQ_POSTING_CAP)) {
    > + struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
    > + struct pi_desc old, new;
    > + unsigned int dest;
    > +
    > + memset(&old, 0, sizeof(old));
    > + memset(&new, 0, sizeof(new));

    This is quite expensive. Just use an u64 for old_control and
    new_control, instead of a full struct.

    >
    > + pi_clear_sn(&new);

    This can be simply new.sn = 0. It does not need atomic operations.

    Same in patch 24 (if needed at all there---see the reply there).

    >
    > + if (irq_remapping_cap(IRQ_POSTING_CAP)) {
    > + struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
    > + struct pi_desc old, new;
    > +
    > + memset(&old, 0, sizeof(old));
    > + memset(&new, 0, sizeof(new));
    > +

    Here you do not need old/new at all because...

    > + if (vcpu->preempted) {
    > + do {
    > + old.control = new.control = pi_desc->control;
    > + pi_set_sn(&new);
    > + } while (cmpxchg(&pi_desc->control, old.control,
    > + new.control) != old.control);

    this can do pi_set_sn directly on pi_desc, without the cmpxchg.

    Paolo


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