linux-kernel - Re: [PATCH v2 1/4] KVM: x86: move the event handling of KVM_REQ_GET_VMCS12

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Date:   Wed, 7 Sep 2022 15:56:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yuan Yao <yuan.yao@...ux.intel.com>
Cc:     Mingwei Zhang <mizhang@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 1/4] KVM: x86: move the event handling of
 KVM_REQ_GET_VMCS12_PAGES into a common function

On Wed, Sep 07, 2022, Sean Christopherson wrote:
> On Wed, Sep 07, 2022, Yuan Yao wrote:
> > On Tue, Sep 06, 2022 at 09:26:33PM -0700, Mingwei Zhang wrote:
> > > > > @@ -10700,6 +10706,12 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
> > > > >               if (kvm_cpu_has_pending_timer(vcpu))
> > > > >                       kvm_inject_pending_timer_irqs(vcpu);
> > > > >
> > > > > +             if (vcpu->arch.nested_get_pages_pending) {
> > > > > +                     r = kvm_get_nested_state_pages(vcpu);
> > > > > +                     if (r <= 0)
> > > > > +                             break;
> > > > > +             }
> > > > > +
> > > >
> > > > Will this leads to skip the get_nested_state_pages for L2 first time
> > > > vmentry in every L2 running iteration ? Because with above changes
> > > > KVM_REQ_GET_NESTED_STATE_PAGES is not set in
> > > > nested_vmx_enter_non_root_mode() and
> > > > vcpu->arch.nested_get_pages_pending is not checked in
> > > > vcpu_enter_guest().
> > > >
> > > Good catch. I think the diff won't work when vcpu is runnable.
> 
> It works, but it's inefficient if the request comes from KVM_SET_NESTED_STATE.
> The pending KVM_REQ_UNBLOCK that comes with the flag will prevent actually running
> the guest.  Specifically, this chunk of code will detect the pending request and
> bail out of vcpu_enter_guest().
> 
> 	if (kvm_vcpu_exit_request(vcpu)) {
> 		vcpu->mode = OUTSIDE_GUEST_MODE;
> 		smp_wmb();
> 		local_irq_enable();
> 		preempt_enable();
> 		kvm_vcpu_srcu_read_lock(vcpu);
> 		r = 1;
> 		goto cancel_injection;
> 	}
> 
> But the inefficiency is a non-issue since "true" emulation of VM-Enter will flow
> through this path (the VMRESUME/VMLAUNCH/VMRUN exit handler runs at the end of
> vcpu_enter_guest().

Actually, nested VM-Enter doesn't use this path at all.  The above holds true for
emulated RSM, but that's largely a moot point since RSM isn't exactly a hot path.