linux-kernel - [PATCH 14/43] KVM: x86: Don't force set BSP bit when local APIC is managed by userspace

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Date:   Fri, 23 Apr 2021 17:46:16 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 14/43] KVM: x86: Don't force set BSP bit when local APIC is
 managed by userspace

Don't set the BSP bit in vcpu->arch.apic_base when the local APIC is
managed by userspace.  Forcing all vCPUs to be BSPs is non-sensical, and
was dead code when it was added by commit 97222cc83163 ("KVM: Emulate
local APIC in kernel").  At the time, kvm_lapic_set_base() was invoked
if and only if the local APIC was in-kernel (and it couldn't be called
before the vCPU created its APIC).

kvm_lapic_set_base() eventually gained generic usage, but the latent bug
escaped notice because the only true consumer would be the guest itself
in the form of an explicit RDMSRs on APs.  Out of Linux, SeaBIOS, and
EDK2/OVMF, only OVMF consume the BSP bit from the APIC_BASE MSR.  For
the vast majority of usage in OVMF, BSP confusion would be benign.
OVMF's BSP election upon SMI rendezvous might be broken, but practically
no one runs KVM with an out-of-kernel local APIC, let alone does so while
utilizing SMIs with OVMF.

Fixes: 97222cc83163 ("KVM: Emulate local APIC in kernel")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/lapic.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b99630c6d7fe..c11f23753a5b 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2252,9 +2252,6 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
 	u64 old_value = vcpu->arch.apic_base;
 	struct kvm_lapic *apic = vcpu->arch.apic;

-	if (!apic)
-		value |= MSR_IA32_APICBASE_BSP;
-
 	vcpu->arch.apic_base = value;

 	if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE)
-- 
2.31.1.498.g6c1eba8ee3d-goog