Re: [PATCH 1/1] KVM: nVMX: update VPPR on vmlaunch/vmresume

[Sean Christopherson]

On Wed, Oct 02, 2024, Sean Christopherson wrote:
> On Wed, Oct 02, 2024, Markku Ahvenjärvi wrote:
> > Hi Sean,
> > 
> > > On Fri, Sep 20, 2024, Markku Ahvenjärvi wrote:
> > > > Running certain hypervisors under KVM on VMX suffered L1 hangs after
> > > > launching a nested guest. The external interrupts were not processed on
> > > > vmlaunch/vmresume due to stale VPPR, and L2 guest would resume without
> > > > allowing L1 hypervisor to process the events.
> > > > 
> > > > The patch ensures VPPR to be updated when checking for pending
> > > > interrupts.
> > >
> > > This is architecturally incorrect, PPR isn't refreshed at VM-Enter.
> > 
> > I looked into this and found the following from Intel manual:
> > 
> > "30.1.3 PPR Virtualization
> > 
> > The processor performs PPR virtualization in response to the following
> > operations: (1) VM entry; (2) TPR virtualization; and (3) EOI virtualization.
> > 
> > ..."
> > 
> > The section "27.3.2.5 Updating Non-Register State" further explains the VM
> > enter:
> > 
> > "If the “virtual-interrupt delivery” VM-execution control is 1, VM entry loads
> > the values of RVI and SVI from the guest interrupt-status field in the VMCS
> > (see Section 25.4.2). After doing so, the logical processor first causes PPR
> > virtualization (Section 30.1.3) and then evaluates pending virtual interrupts
> > (Section 30.2.1). If a virtual interrupt is recognized, it may be delivered in
> > VMX non-root operation immediately after VM entry (including any specified
> > event injection) completes; ..."
> > 
> > According to that, PPR is supposed to be refreshed at VM-Enter, or am I
> > missing something here?
> 
> Huh, I missed that.  It makes sense I guess; VM-Enter processes pending virtual
> interrupts, so it stands that VM-Enter would refresh PPR as well.
> 
> Ugh, and looking again, KVM refreshes PPR every time it checks for a pending
> interrupt, including the VM-Enter case (via kvm_apic_has_interrupt()) when nested
> posted interrupts are in use:
> 
> 	/* Emulate processing of posted interrupts on VM-Enter. */
> 	if (nested_cpu_has_posted_intr(vmcs12) &&
> 	    kvm_apic_has_interrupt(vcpu) == vmx->nested.posted_intr_nv) {
> 		vmx->nested.pi_pending = true;
> 		kvm_make_request(KVM_REQ_EVENT, vcpu);
> 		kvm_apic_clear_irr(vcpu, vmx->nested.posted_intr_nv);
> 	}
> 
> I'm still curious as to what's different about your setup, but certainly not
> curious enough to hold up a fix.

Actually, none of the above is even relevant.  PPR virtualization in the nested
VM-Enter case would be for _L2's_ vPRR, not L1's.  And that virtualization is
performed by hardware (vmcs02 has the correct RVI, SVI, and vAPIC information
for L2).

Which means my initial instinct that KVM is missing a PPR update somewhere is
likely correct.

That said, I'm inclined to go with the below fix anyways, because KVM updates
PPR _constantly_, e.g. every time kvm_vcpu_has_events() is invoked with IRQs
enabled.  Which means that trying to avoid a PPR update on VM-Enter just to be
pedantically accurate is ridiculous.

> So, for an immediate fix, I _think_ we can do:
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index a8e7bc04d9bf..784b61c9810b 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3593,7 +3593,8 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
>          * effectively unblock various events, e.g. INIT/SIPI cause VM-Exit
>          * unconditionally.
>          */
> -       if (unlikely(evaluate_pending_interrupts))
> +       if (unlikely(evaluate_pending_interrupts) ||
> +           kvm_apic_has_interrupt(vcpu))
>                 kvm_make_request(KVM_REQ_EVENT, vcpu);
>  
>         /*