[RFC PATCH 7/7] KVM: x86: Defer tick-based accounting 'til after IRQ handling

[Sean Christopherson <seanjc@google.com>]

When using tick-based accounting, defer the call to account guest time
until after servicing any IRQ(s) that happened in the guest (or
immediately after VM-Exit).  When using tick-based accounting, time is
accounted to the guest when PF_VCPU is set when the tick IRQ handler
runs.  The current approach of unconditionally accounting time in
kvm_guest_exit_irqoff() prevents IRQs that occur in the guest from ever
being processed with PF_VCPU set, since PF_VCPU ends up being set only
during the relatively short VM-Enter sequence, which runs entirely with
IRQs disabled.

Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 8 ++++++++
 arch/x86/kvm/x86.h | 9 ++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 16fb39503296..096bbf50b7a9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9230,6 +9230,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	local_irq_disable();
 	kvm_after_interrupt(vcpu);
 
+	/*
+	 * When using tick-based account, 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.
+	 */
+	if (!IS_ENABLED(CONFIG_VIRT_CPU_ACCOUNTING_GEN))
+		kvm_vtime_account_guest_exit();
+
 	if (lapic_in_kernel(vcpu)) {
 		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
 		if (delta != S64_MIN) {
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 74ef92f47db8..039a7d585925 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -38,15 +38,18 @@ static __always_inline void kvm_guest_exit_irqoff(void)
 	 * 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_irqoff() 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_irqoff();
+
+	if (IS_ENABLED(CONFIG_VIRT_CPU_ACCOUNTING_GEN))
+		kvm_vtime_account_guest_exit();
 
 	instrumentation_begin();
 	trace_hardirqs_off_finish();
-- 
2.31.1.295.g9ea45b61b8-goog