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SubjectRe: [PATCH v2 23/32] KVM: nVMX: Add helper to handle TLB flushes on nested VM-Enter/VM-Exit
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On 17/03/20 19:18, Sean Christopherson wrote:
> On Tue, Mar 17, 2020 at 06:17:59PM +0100, Paolo Bonzini wrote:
>> On 17/03/20 05:52, Sean Christopherson wrote:
>>> + nested_vmx_transition_tlb_flush(vcpu, vmcs12);
>>> +
>>> + /*
>>> + * There is no direct mapping between vpid02 and vpid12, vpid02 is
>>> + * per-vCPU and reused for all nested vCPUs. If vpid12 is changing
>>> + * then the new "virtual" VPID will reuse the same "real" VPID,
>>> + * vpid02, and so needs to be sync'd. Skip the sync if a TLB flush
>>> + * has already been requested, but always update the last used VPID.
>>> + */
>>> + if (nested_cpu_has_vpid(vmcs12) && nested_has_guest_tlb_tag(vcpu) &&
>>> + vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
>>> + vmx->nested.last_vpid = vmcs12->virtual_processor_id;
>>> + if (!kvm_test_request(KVM_REQ_TLB_FLUSH, vcpu))
>>> + vpid_sync_context(nested_get_vpid02(vcpu));
>>> }
>>
>> Would it make sense to move nested_vmx_transition_tlb_flush into an
>> "else" branch?
>
> Maybe? I tried that at one point, but didn't like making the call to
> nested_vmx_transition_tlb_flush() conditional. My intent is to have
> the ...tlb_flush() call be standalone, i.e. logic that is common to all
> nested transitions, so that someone can look at the code can easily
> (relatively speaking) understand the basic rules for TLB flushing on
> nested transitions.

I think it's clear from the above code that we're handling a TLB flush
in a way that doesn't require nested_vmx_transition_tlb_flush. But
perhaps I didn't understand what you mean by "logic that is common to
all nested transitions" and why you named it
nested_vmx_transition_tlb_flush.

Perhaps nested_vmx_transition_tlb_flush could grow a vmentry/vmexit bool
argument instead?

> I also tried the oppositie, i.e. putting the above code in an else-branch,
> with nested_vmx_transition_tlb_flush() returning true if it requested a
> flush. But that required updating vmx->nested.last_vpid in a separate
> flow, which was quite awkward.

No, that's awkward indeed.

Paolo

>> And should this also test that KVM_REQ_TLB_FLUSH_CURRENT is not set?
>
> Doh, yes.
>

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