linux-kernel - [PATCH 6.6.y 09/20] KVM: VMX: Re-enter guest in fastpath for "spurious" preemption timer exits

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Message-ID: <20250815002540.2375664-10-seanjc@google.com>
Date: Thu, 14 Aug 2025 17:25:29 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: stable@...r.kernel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Sasha Levin <sashal@...nel.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 6.6.y 09/20] KVM: VMX: Re-enter guest in fastpath for
 "spurious" preemption timer exits

[ Upstream commit e6b5d16bbd2d4c8259ad76aa33de80d561aba5f9 ]

Re-enter the guest in the fast path if VMX preeemption timer VM-Exit was
"spurious", i.e. if KVM "soft disabled" the timer by writing -1u and by
some miracle the timer expired before any other VM-Exit occurred.  This is
just an intermediate step to cleaning up the preemption timer handling,
optimizing these types of spurious VM-Exits is not interesting as they are
extremely rare/infrequent.

Link: https://lore.kernel.org/r/20240110012705.506918-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ee501871ddb0..32b792387271 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6019,8 +6019,15 @@ static fastpath_t handle_fastpath_preemption_timer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (!vmx->req_immediate_exit &&
-	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
+	/*
+	 * In the *extremely* unlikely scenario that this is a spurious VM-Exit
+	 * due to the timer expiring while it was "soft" disabled, just eat the
+	 * exit and re-enter the guest.
+	 */
+	if (unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled))
+		return EXIT_FASTPATH_REENTER_GUEST;
+
+	if (!vmx->req_immediate_exit) {
 		kvm_lapic_expired_hv_timer(vcpu);
 		return EXIT_FASTPATH_REENTER_GUEST;
 	}
-- 
2.51.0.rc1.163.g2494970778-goog