linux-kernel - [RFC PATCH 1/2] KVM: x86: update APIC_ID also when disabling x2APIC in kvm_lapic_set

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Date:   Mon,  9 Jan 2023 08:06:04 -0500
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        Gautam Menghani <gautammenghani201@...il.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: [RFC PATCH 1/2] KVM: x86: update APIC_ID also when disabling x2APIC in kvm_lapic_set_base

If KVM_SET_MSR firstly enables and then disables x2APIC, make sure
APIC_ID is actually updated correctly, since bits and offset differ from
xAPIC and x2APIC.

Currently this is not handled correctly, as kvm_set_apic_base() will
have msr_info->host_initiated, so switching from x2APIC to xAPIC won't
fail, but kvm_lapic_set_base() does not handle the case.

Fixes: 8d860bbeedef ("kvm: vmx: Basic APIC virtualization controls have three settings")
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
---
 arch/x86/kvm/lapic.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 4efdb4a4d72c..df0a50099aa2 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2394,8 +2394,12 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
 		}
 	}
 
-	if (((old_value ^ value) & X2APIC_ENABLE) && (value & X2APIC_ENABLE))
-		kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);
+	if ((old_value ^ value) & X2APIC_ENABLE) {
+		if (value & X2APIC_ENABLE)
+			kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id);
+		else
+			kvm_apic_set_xapic_id(apic, vcpu->vcpu_id);
+	}
 
 	if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) {
 		kvm_vcpu_update_apicv(vcpu);
-- 
2.31.1