[PATCH 5.10 6/9] KVM: x86: Defer vtime accounting til after IRQ handling

Greg Kroah-Hartman
2021-05-27T15:13:55Z

From: Wanpeng Li <wanpengli@tencent.com>

commit 160457140187c5fb127b844e5a85f87f00a01b14 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/svm/svm.c | 6 +++---
arch/x86/kvm/vmx/vmx.c | 6 +++---
arch/x86/kvm/x86.c | 9 +++++++++
3 files changed, 15 insertions(+), 6 deletions(-)

--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3532,15 +3532,15 @@ static noinstr void svm_vcpu_enter_exit(
* 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();
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6640,15 +6640,15 @@ static noinstr void vmx_vcpu_enter_exit(
* 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();
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9063,6 +9063,15 @@ static int vcpu_enter_guest(struct kvm_v
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) {