linux-kernel - Re: [PATCH 2/5] KVM: x86: do not scan IRR twice on APICv vmentry

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Date:   Fri, 4 Nov 2016 10:38:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        yang.zhang.wz@...il.com, feng.wu@...el.com, mst@...hat.com
Subject: Re: [PATCH 2/5] KVM: x86: do not scan IRR twice on APICv vmentry



On 03/11/2016 21:16, Radim Krčmář wrote:
> > +	if (!pi_test_on(&vmx->pi_desc))
>
> We don't call vmx_hwapic_irr_update() when returning early.

This might be a good start, but it's on purpose: IRR is not changing and
the invariant _should_ be that RVI=highest-bit(IRR):

- IRR cleared by processor: see SDM 29.2.2 Virtual-interrupt delivery

- IRR set by processor: see SDM 29.6 Posted-interrupt processing

- IRR set by KVM: ON=1 so it doesn't exit here

- IRR cleared by KVM: might indeed be buggy here, but the next patch
does add a

+		kvm_x86_ops->hwapic_irr_update(vcpu,
+				apic_find_highest_irr(apic));

to apic_clear_irr, which doesn't fix the bug (and doesn't fix it also if
backported here).


So we're missing a place where IRR has changed but RVI is not being
updated.  It should be related to vmx_check_nested_events and
kvm_cpu_has_interrupt as you said, but I cannot really see it.

Paolo

>> > +		return;
>> > +
>> > +	pi_clear_on(&vmx->pi_desc);
>> > +	max_irr = kvm_apic_update_irr(vcpu, vmx->pi_desc.pir);
>> > +	vmx_hwapic_irr_update(vcpu, max_irr);
>> > +}