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

[Sean Christopherson <seanjc@google.com>]

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@baidu.com>
> ---
>  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.

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