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

Anton Romanov
2022-02-15T20:01:53Z

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