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SubjectRe: [PATCH v3 2/4] KVM: Add paravirt remote TLB flush
On Fri, Nov 10, 2017 at 01:49:45AM -0800, Wanpeng Li wrote:
> +static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
> +
> +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 = src->preempted;

I think that wants to be: state = READ_ONCE(src->preempted);

Because without that its possible for state to get re-loaded between the
check here:

> + if ((state & KVM_VCPU_PREEMPTED)) {

and its use here.

> + if (cmpxchg(&src->preempted, state, state |
> + KVM_VCPU_SHOULD_FLUSH) == state)

You can actually write that like:

if (try_cmpxchg(&src->preempted, state,
state | KVM_VCPU_SHOULD_FLUSH))

Which should generate ever so slightly better code (it uses the cmpxchg
ZF instead of doing a superfluous compare).

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

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