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SubjectRe: [PATCH v4 2/4] KVM: X86: Add paravirt remote TLB flush
On Sun, Nov 12, 2017 at 04:33:24PM -0800, Wanpeng Li wrote:
> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
> + const struct flush_tlb_info *info)
> +{
> + u8 state;
> + int cpu;
> + struct kvm_steal_time *src;
> + struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
> +
> + if (unlikely(!flushmask))
> + return;
> +
> + cpumask_copy(flushmask, cpumask);
> + /*
> + * We have to call flush only on online vCPUs. And
> + * queue flush_on_enter for pre-empted vCPUs
> + */
> + for_each_cpu(cpu, cpumask) {
> + src = &per_cpu(steal_time, cpu);
> + state = READ_ONCE(src->preempted);
> + if ((state & KVM_VCPU_PREEMPTED)) {
> + if (try_cmpxchg(&src->preempted, &state,
> + state | KVM_VCPU_SHOULD_FLUSH))
> + __cpumask_clear_cpu(cpu, flushmask);
> + }
> + }

So if at this point a vCPU gets preempted we'll still spin-wait for it,
which is sub-optimal.

I think we can come up with something to get around that 'problem' if
indeed it is a problem. But we can easily do that as follow up patches.
Just let me know if you think its worth spending more time on.

> + native_flush_tlb_others(flushmask, info);
> +}

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