[PATCH v9 01/21] KVM: x86: Update "last guest TSC" snapshot prior to enabling IRQs/preemption

[Sean Christopherson <seanjc@google.com>]

When refreshing the last observed guest TSC during a guest time update,
write the snapshot before enabling IRQs, i.e. before enabling preemption.
If the task is migrated between updating the local tsc_timestamp, e.g. to
account for catch-up mode, and setting last_guest_tsc, kvm_arch_vcpu_load()
would set the vCPU's TSC offset using the old last_guest_tsc.

In practice, the bug is largely benign as it's not even strictly necessary
for KVM to refresh last_guest_tsc when updating guest time, as KVM's goal
is purely to prevent the guest from observing time jump backwards, i.e.
super duper strictly speaking, KVM only *needs* to update last_guest_tsc in
the VM-Exit path.

In fact, the update kvm_guest_time_update() in wasn't even added to play
nice with kvm_arch_vcpu_load(), it was added by commit 28e4639adf0c ("KVM:
x86: Fix kvmclock bug") to fix code that no longer exists.  As of commit
28e4639adf0c, kvm_guest_time_update() also consumed last_guest_tsc, to try
and prevent guest time from jumping backwards.  That code was eventually
removed by commit f25e656d31ad ("KVM: x86: fix tsc catchup issue with tsc
scaling"), but the last_guest_tsc update hung around.

Keep the update even though it's technically ok to drop the update, e.g. so
that the tsc_catchup updates aren't lost, and so that the guest won't see a
PV clock timestamp that appears to be in the future.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d94b59140c45..d3b47e38698c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1820,6 +1820,12 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
 		}
 	}
 
+	/*
+	 * Refresh L1's last "observed" TSC to match the PV clock's timestamp,
+	 * e.g. so that the guest can't see a TSC that's behind the reference.
+	 */
+	vcpu->last_guest_tsc = tsc_timestamp;
+
 	local_irq_restore(flags);
 
 	/* With all the info we got, fill in the values */
@@ -1841,7 +1847,6 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
 	hv_clock.tsc_to_system_mul = vcpu->pvclock_tsc_mul;
 	hv_clock.tsc_timestamp = tsc_timestamp;
 	hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
-	vcpu->last_guest_tsc = tsc_timestamp;
 
 	/* If the host uses TSC clocksource, then it is stable */
 	hv_clock.flags = 0;
-- 
2.55.0.679.g6767b8d81c-goog