linux-kernel - [PATCH 13/43] KVM: x86: Migrate the PIT only if vcpu0 is migrated, not any BSP

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Date:   Fri, 23 Apr 2021 17:46:15 -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 13/43] KVM: x86: Migrate the PIT only if vcpu0 is migrated,
 not any BSP

Make vcpu0 the arbitrary owner of the PIT, as was intended when PIT
migration was added by commit 2f5997140f22 ("KVM: migrate PIT timer").
The PIT was unintentionally turned into being owned by the BSP by commit
c5af89b68abb ("KVM: Introduce kvm_vcpu_is_bsp() function."), and was then
unintentionally converted to a shared ownership model when
kvm_vcpu_is_bsp() was modified to check the APIC base MSR instead of
hardcoding vcpu0 as the BSP.

Functionally, this just means the PIT's hrtimer is migrated less often.
The real motivation is to remove the usage of kvm_vcpu_is_bsp(), so that
more legacy/broken crud can be removed in a future patch.

Fixes: 58d269d8cccc ("KVM: x86: BSP in MSR_IA32_APICBASE is writable")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/i8254.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index a6e218c6140d..5a69cce4d72d 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -220,7 +220,8 @@ void __kvm_migrate_pit_timer(struct kvm_vcpu *vcpu)
 	struct kvm_pit *pit = vcpu->kvm->arch.vpit;
 	struct hrtimer *timer;

-	if (!kvm_vcpu_is_bsp(vcpu) || !pit)
+	/* Somewhat arbitrarily make vcpu0 the owner of the PIT. */
+	if (vcpu->vcpu_id || !pit)
 		return;

 	timer = &pit->pit_state.timer;
-- 
2.31.1.498.g6c1eba8ee3d-goog