linux-kernel - Re: [PATCH v4 2/4] KVM: X86: Add paravirt remote TLB flush

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Message-ID: <20171113080403.ici7nt5r4b5fpxa6@hirez.programming.kicks-ass.net>
Date:   Mon, 13 Nov 2017 09:04:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [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);
> +}