@@ -11430,16 +11430,18 @@ static void vmx_start_preemption_timer(struct kvm_vcpu *vcpu)
u64 preemption_timeout = get_vmcs12(vcpu)->vmx_preemption_timer_value;
struct vcpu_vmx *vmx = to_vmx(vcpu);
- if (vcpu->arch.virtual_tsc_khz == 0)
- return;
-
- /* Make sure short timeouts reliably trigger an immediate vmexit.
- * hrtimer_start does not guarantee this. */
- if (preemption_timeout <= 1) {
+ /*
+ * A timer value of zero is architecturally guaranteed to cause
+ * a VMExit prior to executing any instructions in the guest.
+ */
+ if (preemption_timeout == 0) {
vmx_preemption_timer_fn(&vmx->nested.preemption_timer);
return;
}
+ if (vcpu->arch.virtual_tsc_khz == 0)
+ return;
+
preemption_timeout <<= VMX_MISC_EMULATED_PREEMPTION_TIMER_RATE;
preemption_timeout *= 1000000;
do_div(preemption_timeout, vcpu->arch.virtual_tsc_khz);
A VMX preemption timer value of '0' at the time of VMEnter is architecturally guaranteed to cause a VMExit prior to the CPU executing any instructions in the guest. This architectural definition is in place to ensure that a previously expired timer is correctly recognized by the CPU as it is possible for the timer to reach zero and not trigger a VMexit due to a higher priority VMExit being signalled instead, e.g. a pending #DB that morphs into a VMExit. Whether by design or coincidence, commit f4124500c2c1 ("KVM: nVMX: Fully emulate preemption timer") special cased timer values of '0' and '1' to ensure prompt delivery of the VMExit. Unlike '0', a timer value of '1' has no has no architectural guarantees regarding when it is delivered. Modify the timer emulation to trigger immediate VMExit if and only if the timer value is '0', and document precisely why '0' is special. Do this even if calibration of the virtual TSC failed, i.e. VMExit will occur immediately regardless of the frequency of the timer. Making only '0' a special case gives KVM leeway to be more aggressive in ensuring the VMExit is injected prior to executing instructions in the nested guest, and also eliminates any ambiguity as to why '1' is a special case, e.g. why wasn't the threshold for a "short timeout" set to 10, 100, 1000, etc... Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/vmx.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-)