[PATCH v2] kvm: x86: Disable KVM_HC_CLOCK_PAIRING if tsc is in always catchup mode

[Anton Romanov <romanton@google.com>]

If vcpu has tsc_always_catchup set each request updates pvclock data.
KVM_HC_CLOCK_PAIRING consumers such as ptp_kvm_x86 rely on tsc read on
host's side and do hypercall inside pvclock_read_retry loop leading to
infinite loop in such situation.

v2:
    Added warn

Signed-off-by: Anton Romanov <romanton@google.com>
---
 arch/x86/kvm/x86.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7131d735b1ef..aaafb46a6048 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8945,6 +8945,15 @@ static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
 	if (!kvm_get_walltime_and_clockread(&ts, &cycle))
 		return -KVM_EOPNOTSUPP;
 
+	/*
+	 * When tsc is in permanent catchup mode guests won't be able to use
+	 * pvclock_read_retry loop to get consistent view of pvclock
+	 */
+	if (vcpu->arch.tsc_always_catchup) {
+		pr_warn_ratelimited("KVM_HC_CLOCK_PAIRING not supported if vcpu is in tsc catchup mode\n");
+		return -KVM_EOPNOTSUPP;
+	}
+
 	clock_pairing.sec = ts.tv_sec;
 	clock_pairing.nsec = ts.tv_nsec;
 	clock_pairing.tsc = kvm_read_l1_tsc(vcpu, cycle);
-- 
2.35.1.265.g69c8d7142f-goog