Re: [PATCH v3 2/4] KVM: Add paravirt remote TLB flush

[Peter Zijlstra]

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);
> +}