Re: [PATCH] KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself

[Paolo Bonzini]

On 12/2/22 16:49, Sean Christopherson wrote:
> On Fri, Dec 02, 2022, Yuan ZhaoXiong wrote:
> > This patch fixes that VM rebooting itself will cause APICv
> > disabled when VM is started with APICv/x2AVIC enabled.
> >
> > When a VM reboot itself, The Qemu whill reset LAPIC by invoking
> > ioctl(KVM_SET_LAPIC, ...) to disable x2APIC mode and set APIC_ID
> > to its vcpuid in xAPIC mode.
> >
> > That will be handled in KVM as follows:
> >
> >       kvm_vcpu_ioctl_set_lapic
> >         kvm_apic_set_state
> > 	  kvm_lapic_set_base  =>  disable X2APIC mode
> > 	    kvm_apic_state_fixup
> > 	      kvm_lapic_xapic_id_updated
> > 	        kvm_xapic_id(apic) != apic->vcpu->vcpu_id
> > 		kvm_set_apicv_inhibit(APICV_INHIBIT_REASON_APIC_ID_MODIFIED)
> > 	   memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s))  => update APIC_ID
> >
> > kvm_apic_set_state invokes kvm_lapic_set_base to disable x2APIC mode
> > firstly, but don't change APIC_ID, APIC_ID is 32 bits in x2APIC mode
> > and 8 bist(bit 24 ~ bit 31) in xAPIC mode. So kvm_lapic_xapic_id_updated
> > will set APICV_INHIBIT_REASON_APIC_ID_MODIFIED bit inhibit and disable
> > APICv/x2AVIC.
> >
> > kvm_lapic_xapic_id_updated must be called after APIC_ID is changed.
> >
> > Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
> >
> > Signed-off-by: Yuan ZhaoXiong <yuanzhaoxiong@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/lapic.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index d7639d1..bf5ce86 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -2722,8 +2722,6 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
> >   			icr = __kvm_lapic_get_reg64(s->regs, APIC_ICR);
> >   			__kvm_lapic_set_reg(s->regs, APIC_ICR2, icr >> 32);
> >   		}
> > -	} else {
> > -		kvm_lapic_xapic_id_updated(vcpu->arch.apic);
> >   	}
> >   
> >   	return 0;
> > @@ -2759,6 +2757,9 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
> >   	}
> >   	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
> >   
> > +	if (!apic_x2apic_mode(apic))
> > +		kvm_lapic_xapic_id_updated(apic);
> > +
>
> Already posted[*], along with a pile of other APIC fixes.  Hopefully it will land
> in 6.2.

I'll let this patch overtake the others since it is stable@ material.

I'll also volunteer someone to write a testcase (we can add an 
apicv_enabled boolean statistic too for the sake of the test).

Paolo

> [*] https://lore.kernel.org/all/20221001005915.2041642-7-seanjc@xxxxxxxxxx