From 1fc288e74cf31c700cfadf30e39867fec8da541b Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Thu, 14 Aug 2025 17:25:29 -0700 Subject: [PATCH] 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 Signed-off-by: Sasha Levin --- 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 ee501871ddb03..32b7923872712 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; } -- 2.47.3