KVM: x86: Defer vtime accounting 'til after IRQ handling

Defer the call to account guest time until after servicing any IRQ(s)
that happened in the guest or immediately after VM-Exit.  Tick-based
accounting of vCPU time relies on PF_VCPU being set when the tick IRQ
handler runs, and IRQs are blocked throughout the main sequence of
vcpu_enter_guest(), including the call into vendor code to actually
enter and exit the guest.

This fixes a bug where reported guest time remains '0', even when
running an infinite loop in the guest:

  https://bugzilla.kernel.org/show_bug.cgi?id=209831

Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210505002735.1684165-4-seanjc@google.com

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 9790c73..c400def 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3753,15 +3753,15 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
         * have them in state 'on' as recorded before entering guest mode.
         * Same as enter_from_user_mode().
         *
-        * guest_exit_irqoff() restores host context and reinstates RCU if
-        * enabled and required.
+        * context_tracking_guest_exit() restores host context and reinstates
+        * RCU if enabled and required.
         *
         * This needs to be done before the below as native_read_msr()
         * contains a tracepoint and x86_spec_ctrl_restore_host() calls
         * into world and some more.
         */
        lockdep_hardirqs_off(CALLER_ADDR0);
-       guest_exit_irqoff();
+       context_tracking_guest_exit();
 
        instrumentation_begin();
        trace_hardirqs_off_finish();

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b21d751..e108fb4 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6703,15 +6703,15 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
         * have them in state 'on' as recorded before entering guest mode.
         * Same as enter_from_user_mode().
         *
-        * guest_exit_irqoff() restores host context and reinstates RCU if
-        * enabled and required.
+        * context_tracking_guest_exit() restores host context and reinstates
+        * RCU if enabled and required.
         *
         * This needs to be done before the below as native_read_msr()
         * contains a tracepoint and x86_spec_ctrl_restore_host() calls
         * into world and some more.
         */
        lockdep_hardirqs_off(CALLER_ADDR0);
-       guest_exit_irqoff();
+       context_tracking_guest_exit();
 
        instrumentation_begin();
        trace_hardirqs_off_finish();

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cebdaa1..6eda283 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9315,6 +9315,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
        local_irq_disable();
        kvm_after_interrupt(vcpu);
 
+       /*
+        * Wait until after servicing IRQs to account guest time so that any
+        * ticks that occurred while running the guest are properly accounted
+        * to the guest.  Waiting until IRQs are enabled degrades the accuracy
+        * of accounting via context tracking, but the loss of accuracy is
+        * acceptable for all known use cases.
+        */
+       vtime_account_guest_exit();
+
        if (lapic_in_kernel(vcpu)) {
                s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
                if (delta != S64_MIN) {