[PATCH v4 09/30] KVM: x86: WARN if kvm_get_walltime_and_clockread() fails unexpectedly

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

The master clock depends on the pvclock being based on TSC, so the only
way kvm_get_walltime_and_clockread() can fail when use_master_clock is
true is if the clocksource changed and use_master_clock is stale, in
which case a seqcount retry should be pending.

Add a WARN_ON_ONCE if the seqcount has not been invalidated, to catch
any case where the failure is genuinely unexpected.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1e1834533e98..ccdfd3fa3402 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3474,7 +3474,8 @@ uint64_t kvm_get_wall_clock_epoch(struct kvm *kvm)
 		local_tsc_khz = get_cpu_tsc_khz();
 
 		if (local_tsc_khz &&
-		    !kvm_get_walltime_and_clockread(&ts, &host_tsc))
+		    !kvm_get_walltime_and_clockread(&ts, &host_tsc) &&
+		    WARN_ON_ONCE(!read_seqcount_retry(&ka->pvclock_sc, seq)))
 			local_tsc_khz = 0; /* Fall back to old method */
 
 		put_cpu();
-- 
2.51.0