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

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Date:   Mon, 13 Nov 2017 16:26:57 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <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

2017-11-13 16:04 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> 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.

You can post your idea, it is always smart. :) Then we can evaluate
the complexity and gains.

Regards,
Wanpeng Li