[RFC PATCH] KVM: x86: Set PF_VCPU when processing IRQs to fix tick-based accounting

[Sean Christopherson]

Temporarily set PF_VCPU while processing IRQ VM-Exits so that a tick IRQ
accounts the time slice to the guest.  Tick-based accounting of guest
time is currently broken as PF_VCPU is only set during the relatively
short VM-Enter sequence, which runs entirely with IRQs disabled, and IRQs
that occur in the guest are processed well after PF_VCPU is cleared.

Keep PF_VCPU set across both VMX's processing of already-acked IRQs
(handle_exit_irqoff()) and the explicit IRQ window (SVM's processing,
plus ticks that occur immediately after VM-Exit on both VMX and SVM).

Fixes: 87fa7f3e98a1 ("x86/kvm: Move context tracking where it belongs")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---

This is quite obnoxious, hence the RFC, but I can't think of a clever,
less ugly way to fix the accounting.

 arch/x86/kvm/x86.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d9f931c63293..6ddf341cd755 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9118,6 +9118,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	vcpu->mode = OUTSIDE_GUEST_MODE;
 	smp_wmb();
 
+	/*
+	 * Temporarily pretend this task is running a vCPU when potentially
+	 * processing an IRQ exit, including the below opening of an IRQ
+	 * window.  Tick-based accounting of guest time relies on PF_VCPU
+	 * being set when the tick IRQ handler runs.
+	 */
+	current->flags |= PF_VCPU;
 	static_call(kvm_x86_handle_exit_irqoff)(vcpu);
 
 	/*
@@ -9132,6 +9139,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	++vcpu->stat.exits;
 	local_irq_disable();
 	kvm_after_interrupt(vcpu);
+	current->flags &= ~PF_VCPU;
 
 	if (lapic_in_kernel(vcpu)) {
 		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
-- 
2.30.0.478.g8a0d178c01-goog