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SubjectRe: [PATCH v4 2/4] KVM: X86: Add paravirt remote TLB flush
On Mon, Nov 13, 2017 at 04:26:57PM +0800, Wanpeng Li wrote:
> 2017-11-13 16:04 GMT+08:00 Peter Zijlstra <peterz@infradead.org>:

> > 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.

I'm not sure I have a fully baked idea just yet, but the general idea
would be something like:

- switch (back) to a dedicated TLB invalidate IPI

- introduce KVM_VCPU_IPI_PENDING

- change flush_tlb_others() into something like:

for_each_cpu(cpu, flushmask) {
src = &per_cpu(steal_time, cpu);
state = READ_ONCE(src->preempted);
do {
if (state & KVM_VCPU_PREEMPTED) {
if (try_cmpxchg(&src->preempted, &state,
state | KVM_VCPU_SHOULD_FLUSH)) {
__cpumask_clear_cpu(cpu, flushmask);
break;
}
}
} while (!try_cmpxchg(&src->preempted, &state,
state | KVM_VCPU_IPI_PENDING));
}

apic->send_IPI_mask(flushmask, CALL_TLB_VECTOR);

for_each_cpu(cpu, flushmask) {
src = &per_cpu(steal_time, cpu);
smp_cond_load_acquire(&src->preempted, !(VAL & KVM_VCPU_IPI_PENDING);
}


- have the TLB invalidate handler do something like:

state = READ_ONCE(src->preempted);
if (!(state & KVM_VCPU_IPI_PENDING))
return;

local_flush_tlb();

do {
} while (!try_cmpxchg(&src->preempted, &state,
state & ~KVM_VCPU_IPI_PENDING));

- then at VMEXIT time do something like:

state = READ_ONCE(src->preempted);
do {
if (!(state & KVM_VCPU_IPI_PENDING))
break;
} while (!try_cmpxchg(&src->preempted, state,
(state & ~KVM_VCPU_IPI_PENDING) |
KVM_VCPU_SHOULD_FLUSH));

and clear any possible pending TLB_VECTOR in the guest state to avoid
raising that IPI spuriously on enter again.


This way the preemption will clear the IPI_PENDING and the
flush_others() wait loop will terminate.

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