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SubjectRe: [PATCH v3 4/4] KVM: Add flush_on_enter before guest enter
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On 10/11/2017 11:08, Peter Zijlstra wrote:
> On Fri, Nov 10, 2017 at 01:49:47AM -0800, Wanpeng Li wrote:
>> @@ -2887,7 +2899,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>> if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>> return;
>>
>> - vcpu->arch.st.steal.preempted = KVM_VCPU_PREEMPTED;
>> + vcpu->arch.st.steal.preempted |= KVM_VCPU_PREEMPTED;
>
> I don't understand this one... If there is concurrency its wrong, if
> there is no concurrency it still doesn't make sense as there should not
> be any FLUSH flags to preserve..

There is no concurrency, foreign VCPUs are not going to write to the
location unless PREEMPTED is set. So indeed the "|=" is pointless.

However, I wonder if it'd be useful for a VCPU to set the bit _on
itself_ before going to sleep. Like

set KVM_VCPU_SHOULD_FLUSH
hlt
/* Automagic TLB flush! */

This would not work currently, but if it *is* useful, we should make it
work and document it as legal. Peter, do you think it would make any sense?

Thanks,

Paolo


>>
>> kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime,
>> &vcpu->arch.st.steal.preempted,

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