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@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0b495979a02bc3d9c61e4a122fc44bd9e0ab1ffb..96bbccd9477c4b9cd79a86a5f62773aa8cb40382 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5933,8 +5933,15 @@ 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;
        }