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

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Date:   Fri,  2 Dec 2022 20:36:14 +0800
From:   Yuan ZhaoXiong <yuanzhaoxiong@...du.com>
To:     seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mlevitsk@...hat.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself

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@...du.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);
+
 	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
 	kvm_recalculate_apic_map(vcpu->kvm);
 	kvm_apic_set_version(vcpu);
-- 
1.8.3.1