[PATCH v3] KVM: x86/tsc: Don't sync user changes to TSC with KVM-initiated change

[Like Xu]

From: Like Xu <likexu@xxxxxxxxxxx>

Add kvm->arch.user_changed_tsc to avoid synchronizing user changes to
the TSC with the KVM-initiated change in kvm_arch_vcpu_postcreate() by
conditioning this mess on userspace having written the TSC at least
once already.

Here lies UAPI baggage: user-initiated TSC write with a small delta
(1 second) of virtual cycle time against real time is interpreted as an
attempt to synchronize the CPU. In such a scenario, the vcpu's tsc_offset
is not configured as expected, resulting in significant guest service
response latency, which is observed in our production environment.

Reported-by: Yong He <alexyonghe@xxxxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Suggested-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Original-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Tested-by: Like Xu <likexu@xxxxxxxxxxx>
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
V2 -> V3 Changelog:
- Use the kvm->arch.user_changed_tsc proposal; (Oliver & Paolo)
V2: https://lore.kernel.org/kvm/20230724073516.45394-1-likexu@xxxxxxxxxxx/

 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/x86.c              | 23 ++++++++++++++++-------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3bc146dfd38d..e8d423ef1474 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1303,6 +1303,7 @@ struct kvm_arch {
 	u64 cur_tsc_offset;
 	u64 cur_tsc_generation;
 	int nr_vcpus_matched_tsc;
+	bool user_changed_tsc;
 
 	u32 default_tsc_khz;
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 278dbd37dab2..eeaf4ad9174d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2713,7 +2713,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
 	kvm_track_tsc_matching(vcpu);
 }
 
-static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
+static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data, bool user_initiated)
 {
 	struct kvm *kvm = vcpu->kvm;
 	u64 offset, ns, elapsed;
@@ -2734,20 +2734,29 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 			 * kvm_clock stable after CPU hotplug
 			 */
 			synchronizing = true;
-		} else {
+		} else if (kvm->arch.user_changed_tsc) {
 			u64 tsc_exp = kvm->arch.last_tsc_write +
 						nsec_to_cycles(vcpu, elapsed);
 			u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
 			/*
-			 * Special case: TSC write with a small delta (1 second)
-			 * of virtual cycle time against real time is
-			 * interpreted as an attempt to synchronize the CPU.
+			 * Here lies UAPI baggage: user-initiated TSC write with
+			 * a small delta (1 second) of virtual cycle time
+			 * against real time is interpreted as an attempt to
+			 * synchronize the CPU.
+			 *
+			 * Don't synchronize user changes to the TSC with the
+			 * KVM-initiated change in kvm_arch_vcpu_postcreate()
+			 * by conditioning this mess on userspace having
+			 * written the TSC at least once already.
 			 */
 			synchronizing = data < tsc_exp + tsc_hz &&
 					data + tsc_hz > tsc_exp;
 		}
 	}
 
+	if (user_initiated)
+		kvm->arch.user_changed_tsc = true;
+
 	/*
 	 * For a reliable TSC, we can match TSC offsets, and for an unstable
 	 * TSC, we add elapsed time in this computation.  We could let the
@@ -3776,7 +3785,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_IA32_TSC:
 		if (msr_info->host_initiated) {
-			kvm_synchronize_tsc(vcpu, data);
+			kvm_synchronize_tsc(vcpu, data, true);
 		} else {
 			u64 adj = kvm_compute_l1_tsc_offset(vcpu, data) - vcpu->arch.l1_tsc_offset;
 			adjust_tsc_offset_guest(vcpu, adj);
@@ -11950,7 +11959,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 	if (mutex_lock_killable(&vcpu->mutex))
 		return;
 	vcpu_load(vcpu);
-	kvm_synchronize_tsc(vcpu, 0);
+	kvm_synchronize_tsc(vcpu, 0, false);
 	vcpu_put(vcpu);
 
 	/* poll control enabled by default */

base-commit: 5a7591176c47cce363c1eed704241e5d1c42c5a6
-- 
2.41.0