linux-kernel - Re: [PATCH] x86/kvm/vmx: Don't halt vcpu when L1 is injecting events to L2

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Date:   Fri, 9 Feb 2018 18:16:39 -0800 (PST)
From:   Liran Alon <liran.alon@...cle.com>
To:     <pbonzini@...hat.com>
Cc:     <mingo@...hat.com>, <chao.gao@...el.com>, <x86@...nel.org>,
        <tglx@...utronix.de>, <rkrcmar@...hat.com>,
        <linux-kernel@...r.kernel.org>, <hpa@...or.com>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86/kvm/vmx: Don't halt vcpu when L1 is injecting events
 to L2


----- pbonzini@...hat.com wrote:

> On 08/02/2018 13:09, Liran Alon wrote:
> > ----- pbonzini@...hat.com wrote:
> >> On 08/02/2018 06:13, Chao Gao wrote:
> > 
> > A possible patch to fix this is to change vmx_hwapic_irr_update()
> such that
> > if is_guest_mode(vcpu)==true, we should return max(max_irr, rvi) and
> return
> > that value into apic_has_interrupt_for_ppr().
> > Need to verify that it doesn't break other flows but I think it
> makes sense.
> > What do you think?
> 
> Yeah, I think it makes sense though I'd need to look a lot more at
> arch/x86/kvm/lapic.c and arch/x86/kvm/vmx.c to turn that into a
> patch!
> 
> Paolo

After thinking about this a bit more, I don't like my previous suggestion.
As we don't semantically want to change the value returned from kvm_apic_has_interrupt().
Instead, it makes more sense to change kvm_cpu_has_interrupt() to check for RVI>PPR
in case is_guest_mode(vcpu)==true.

Something like (partial theoretical patch):

@@ -97,6 +97,14 @@ int kvm_cpu_has_interrupt(struct kvm_vcpu *v)
        if (kvm_cpu_has_extint(v))
                return 1;
 
+       /*
+        * When running L2, L1 controls vmcs02 RVI via vmcs12.
+        * Therefore, it is possible RVI indicates pending interrupt
+        * for vCPU while LAPIC IRR is empty.
+        */
+       if (is_guest_mode(v) &&
+           (kvm_x86_ops->hwapic_has_interrupt(v) != -1))
+               return 1;
+
        return kvm_apic_has_interrupt(v) != -1; /* LAPIC */
 }

Where:

+static int vmx_get_rvi(void)
+{
+       return ((u8)vmcs_read16(GUEST_INTR_STATUS) & 0xff);
+}

+static int vmx_hwapic_has_interrupt(struct kvm_vcpu *vcpu)
+{
+       int vector = vmx_get_rvi(vcpu);
+       return kvm_apic_has_interrupt_for_vector(vector);
+}

+int kvm_apic_has_interrupt_for_vector(struct kvm_vcpu *vcpu, int vector)
+{
+       struct kvm_lapic *apic = vcpu->arch.apic;
+       u32 ppr;
+
+       if (!apic_enabled(apic))
+               return -1;
+
+       __apic_update_ppr(apic, &ppr);
+       return (((vector & 0xF0) > ppr) ? (vector) : (-1));
+}
+EXPORT_SYMBOL_GPL(kvm_apic_has_interrupt_for_vector);

Regards,
-Liran