linux-kernel - Re: [PATCH v3 3/7] KVM: nVMX: Make an event request when pending an MTF nested VM-Exit

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Date:   Mon, 22 Aug 2022 21:40:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        mlevitsk@...hat.com, seanjc@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v3 3/7] KVM: nVMX: Make an event request when pending an
 MTF nested VM-Exit

On 8/22/22 19:52, Jim Mattson wrote:
> On Mon, Aug 22, 2022 at 10:08 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> From: Sean Christopherson <seanjc@...gle.com>
>>
>> Set KVM_REQ_EVENT when MTF becomes pending to ensure that KVM will run
>> through inject_pending_event() and thus vmx_check_nested_events() prior
>> to re-entering the guest.
>>
>> MTF currently works by virtue of KVM's hack that calls
>> kvm_check_nested_events() from kvm_vcpu_running(), but that hack will
>> be removed in the near future.  Until that call is removed, the patch
>> introduces no functional change.
>>
>> Fixes: 5ef8acbdd687 ("KVM: nVMX: Emulate MTF when performing instruction emulation")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
> What happens if live migration occurs before the KVM_REQ_EVENT is processed?

Good point, this must be squashed in:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index ab135f9ef52f..a703b71d675d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6481,6 +6481,8 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
  	if (ret)
  		goto error_guest_mode;
  
+	if (vmx->nested.mtf_pending)
+		kvm_make_request(KVM_REQ_EVENT, vcpu);
  	return 0;
  
  error_guest_mode:

so that the other side restores the request.

Paolo