KVM: x86: hyper-v: Track Hyper-V TSC page status

Create an infrastructure for tracking Hyper-V TSC page status, i.e. if it
was updated from guest/host side or if we've failed to set it up (because
e.g. guest wrote some garbage to HV_X64_MSR_REFERENCE_TSC) and there's no
need to retry.

Also, in a hypothetical situation when we are in 'always catchup' mode for
TSC we can now avoid contending 'hv->hv_lock' on every guest enter by
setting the state to HV_TSC_PAGE_BROKEN after compute_tsc_page_parameters()
returns false.

Check for HV_TSC_PAGE_SET state instead of '!hv->tsc_ref.tsc_sequence' in
get_time_ref_counter() to properly handle the situation when we failed to
write the updated TSC page values to the guest.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210316143736.964151-4-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -884,12 +884,29 @@ struct kvm_hv_syndbg {
 	u64 options;
 };
 
+/* Current state of Hyper-V TSC page clocksource */
+enum hv_tsc_page_status {
+	/* TSC page was not set up or disabled */
+	HV_TSC_PAGE_UNSET = 0,
+	/* TSC page MSR was written by the guest, update pending */
+	HV_TSC_PAGE_GUEST_CHANGED,
+	/* TSC page MSR was written by KVM userspace, update pending */
+	HV_TSC_PAGE_HOST_CHANGED,
+	/* TSC page was properly set up and is currently active  */
+	HV_TSC_PAGE_SET,
+	/* TSC page is currently being updated and therefore is inactive */
+	HV_TSC_PAGE_UPDATING,
+	/* TSC page was set up with an inaccessible GPA */
+	HV_TSC_PAGE_BROKEN,
+};
+
 /* Hyper-V emulation context */
 struct kvm_hv {
 	struct mutex hv_lock;
 	u64 hv_guest_os_id;
 	u64 hv_hypercall;
 	u64 hv_tsc_page;
+	enum hv_tsc_page_status hv_tsc_page_status;
 
 	/* Hyper-v based guest crash (NT kernel bugcheck) parameters */
 	u64 hv_crash_param[HV_X64_MSR_CRASH_PARAMS];

arch/x86/kvm/hyperv.c

@@ -520,10 +520,10 @@ static u64 get_time_ref_counter(struct kvm *kvm)
 	u64 tsc;
 
 	/*
-	 * The guest has not set up the TSC page or the clock isn't
-	 * stable, fall back to get_kvmclock_ns.
+	 * Fall back to get_kvmclock_ns() when TSC page hasn't been set up,
+	 * is broken, disabled or being updated.
 	 */
-	if (!hv->tsc_ref.tsc_sequence)
+	if (hv->hv_tsc_page_status != HV_TSC_PAGE_SET)
 		return div_u64(get_kvmclock_ns(kvm), 100);
 
 	vcpu = kvm_get_vcpu(kvm, 0);
@@ -1087,7 +1087,8 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
 	BUILD_BUG_ON(sizeof(tsc_seq) != sizeof(hv->tsc_ref.tsc_sequence));
 	BUILD_BUG_ON(offsetof(struct ms_hyperv_tsc_page, tsc_sequence) != 0);
 
-	if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
+	if (hv->hv_tsc_page_status == HV_TSC_PAGE_BROKEN ||
+	    hv->hv_tsc_page_status == HV_TSC_PAGE_UNSET)
 		return;
 
 	mutex_lock(&hv->hv_lock);
@@ -1101,7 +1102,7 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
 	 */
 	if (unlikely(kvm_read_guest(kvm, gfn_to_gpa(gfn),
 				    &tsc_seq, sizeof(tsc_seq))))
-		goto out_unlock;
+		goto out_err;
 
 	/*
 	 * While we're computing and writing the parameters, force the
@@ -1110,15 +1111,15 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
 	hv->tsc_ref.tsc_sequence = 0;
 	if (kvm_write_guest(kvm, gfn_to_gpa(gfn),
 			    &hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence)))
-		goto out_unlock;
+		goto out_err;
 
 	if (!compute_tsc_page_parameters(hv_clock, &hv->tsc_ref))
-		goto out_unlock;
+		goto out_err;
 
 	/* Ensure sequence is zero before writing the rest of the struct.  */
 	smp_wmb();
 	if (kvm_write_guest(kvm, gfn_to_gpa(gfn), &hv->tsc_ref, sizeof(hv->tsc_ref)))
-		goto out_unlock;
+		goto out_err;
 
 	/*
 	 * Now switch to the TSC page mechanism by writing the sequence.
@@ -1131,8 +1132,15 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
 	smp_wmb();
 
 	hv->tsc_ref.tsc_sequence = tsc_seq;
-	kvm_write_guest(kvm, gfn_to_gpa(gfn),
-			&hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence));
+	if (kvm_write_guest(kvm, gfn_to_gpa(gfn),
+			    &hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence)))
+		goto out_err;
+
+	hv->hv_tsc_page_status = HV_TSC_PAGE_SET;
+	goto out_unlock;
+
+out_err:
+	hv->hv_tsc_page_status = HV_TSC_PAGE_BROKEN;
 out_unlock:
 	mutex_unlock(&hv->hv_lock);
 }
@@ -1142,7 +1150,8 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm)
 	struct kvm_hv *hv = to_kvm_hv(kvm);
 	u64 gfn;
 
-	if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
+	if (hv->hv_tsc_page_status == HV_TSC_PAGE_BROKEN ||
+	    hv->hv_tsc_page_status == HV_TSC_PAGE_UNSET)
 		return;
 
 	mutex_lock(&hv->hv_lock);
@@ -1150,11 +1159,16 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm)
 	if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
 		goto out_unlock;
 
+	/* Preserve HV_TSC_PAGE_GUEST_CHANGED/HV_TSC_PAGE_HOST_CHANGED states */
+	if (hv->hv_tsc_page_status == HV_TSC_PAGE_SET)
+		hv->hv_tsc_page_status = HV_TSC_PAGE_UPDATING;
+
 	gfn = hv->hv_tsc_page >> HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT;
 
 	hv->tsc_ref.tsc_sequence = 0;
-	kvm_write_guest(kvm, gfn_to_gpa(gfn),
-			&hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence));
+	if (kvm_write_guest(kvm, gfn_to_gpa(gfn),
+			    &hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence)))
+		hv->hv_tsc_page_status = HV_TSC_PAGE_BROKEN;
 
 out_unlock:
 	mutex_unlock(&hv->hv_lock);
@@ -1216,8 +1230,15 @@ static int kvm_hv_set_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data,
 	}
 	case HV_X64_MSR_REFERENCE_TSC:
 		hv->hv_tsc_page = data;
-		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE)
+		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE) {
+			if (!host)
+				hv->hv_tsc_page_status = HV_TSC_PAGE_GUEST_CHANGED;
+			else
+				hv->hv_tsc_page_status = HV_TSC_PAGE_HOST_CHANGED;
 			kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
+		} else {
+			hv->hv_tsc_page_status = HV_TSC_PAGE_UNSET;
+		}
 		break;
 	case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
 		return kvm_hv_msr_set_crash_data(kvm,