KVM: x86/xen: Allow XEN_RUNSTATE_UPDATE flag behaviour to be configured

Closer inspection of the Xen code shows that we aren't supposed to be
using the XEN_RUNSTATE_UPDATE flag unconditionally. It should be
explicitly enabled by guests through the HYPERVISOR_vm_assist hypercall.
If we randomly set the top bit of ->state_entry_time for a guest that
hasn't asked for it and doesn't expect it, that could make the runtimes
fail to add up and confuse the guest. Without the flag it's perfectly
safe for a vCPU to read its own vcpu_runstate_info; just not for one
vCPU to read *another's*.

I briefly pondered adding a word for the whole set of VMASST_TYPE_*
flags but the only one we care about for HVM guests is this, so it
seemed a bit pointless.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20221127122210.248427-3-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -5339,6 +5339,7 @@ KVM_PV_ASYNC_CLEANUP_PERFORM
 	union {
 		__u8 long_mode;
 		__u8 vector;
+		__u8 runstate_update_flag;
 		struct {
 			__u64 gfn;
 		} shared_info;
@@ -5416,6 +5417,14 @@ KVM_XEN_ATTR_TYPE_XEN_VERSION
   event channel delivery, so responding within the kernel without
   exiting to userspace is beneficial.
 
+KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG
+  This attribute is available when the KVM_CAP_XEN_HVM ioctl indicates
+  support for KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG. It enables the
+  XEN_RUNSTATE_UPDATE flag which allows guest vCPUs to safely read
+  other vCPUs' vcpu_runstate_info. Xen guests enable this feature via
+  the VM_ASST_TYPE_runstate_update_flag of the HYPERVISOR_vm_assist
+  hypercall.
+
 4.127 KVM_XEN_HVM_GET_ATTR
 --------------------------
 
@@ -8065,6 +8074,7 @@ PVHVM guests. Valid flags are::
   #define KVM_XEN_HVM_CONFIG_RUNSTATE			(1 << 3)
   #define KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL		(1 << 4)
   #define KVM_XEN_HVM_CONFIG_EVTCHN_SEND		(1 << 5)
+  #define KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG	(1 << 6)
 
 The KVM_XEN_HVM_CONFIG_HYPERCALL_MSR flag indicates that the KVM_XEN_HVM_CONFIG
 ioctl is available, for the guest to set its hypercall page.
@@ -8096,6 +8106,18 @@ KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID/TIMER/UPCALL_VECTOR vCPU attributes.
 related to event channel delivery, timers, and the XENVER_version
 interception.
 
+The KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG flag indicates that KVM supports
+the KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG attribute in the KVM_XEN_SET_ATTR
+and KVM_XEN_GET_ATTR ioctls. This controls whether KVM will set the
+XEN_RUNSTATE_UPDATE flag in guest memory mapped vcpu_runstate_info during
+updates of the runstate information. Note that versions of KVM which support
+the RUNSTATE feature above, but not thie RUNSTATE_UPDATE_FLAG feature, will
+always set the XEN_RUNSTATE_UPDATE flag when updating the guest structure,
+which is perhaps counterintuitive. When this flag is advertised, KVM will
+behave more correctly, not using the XEN_RUNSTATE_UPDATE flag until/unless
+specifically enabled (by the guest making the hypercall, causing the VMM
+to enable the KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG attribute).
+
 8.31 KVM_CAP_PPC_MULTITCE
 -------------------------
 

arch/x86/include/asm/kvm_host.h

@@ -1113,6 +1113,7 @@ struct msr_bitmap_range {
 struct kvm_xen {
 	u32 xen_version;
 	bool long_mode;
+	bool runstate_update_flag;
 	u8 upcall_vector;
 	struct gfn_to_pfn_cache shinfo_cache;
 	struct idr evtchn_ports;

arch/x86/kvm/x86.c

@@ -4431,7 +4431,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		    KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL |
 		    KVM_XEN_HVM_CONFIG_EVTCHN_SEND;
 		if (sched_info_on())
-			r |= KVM_XEN_HVM_CONFIG_RUNSTATE;
+			r |= KVM_XEN_HVM_CONFIG_RUNSTATE |
+			     KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG;
 		break;
 #endif
 	case KVM_CAP_SYNC_REGS:

arch/x86/kvm/xen.c

@@ -179,7 +179,8 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 	struct vcpu_runstate_info rs;
 	unsigned long flags;
 	size_t times_ofs;
-	uint8_t *update_bit;
+	uint8_t *update_bit = NULL;
+	uint64_t entry_time;
 	uint64_t *rs_times;
 	int *rs_state;
 
@@ -297,6 +298,7 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 		 */
 		rs_state = gpc1->khva;
 		rs_times = gpc1->khva + times_ofs;
+		if (v->kvm->arch.xen.runstate_update_flag)
 			update_bit = ((void *)(&rs_times[1])) - 1;
 	} else {
 		/*
@@ -351,12 +353,14 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 		 * The update_bit is still directly in the guest memory,
 		 * via one GPC or the other.
 		 */
+		if (v->kvm->arch.xen.runstate_update_flag) {
 			if (user_len1 >= times_ofs + sizeof(uint64_t))
 				update_bit = gpc1->khva + times_ofs +
 					sizeof(uint64_t) - 1;
 			else
 				update_bit = gpc2->khva + times_ofs +
 					sizeof(uint64_t) - 1 - user_len1;
+		}
 
 #ifdef CONFIG_X86_64
 		/*
@@ -376,8 +380,12 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 	 * different cache line to the rest of the 64-bit word, due to
 	 * the (lack of) alignment constraints.
 	 */
+	entry_time = vx->runstate_entry_time;
+	if (update_bit) {
+		entry_time |= XEN_RUNSTATE_UPDATE;
 		*update_bit = (vx->runstate_entry_time | XEN_RUNSTATE_UPDATE) >> 56;
 		smp_wmb();
+	}
 
 	/*
 	 * Now assemble the actual structure, either on our kernel stack
@@ -385,7 +393,7 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 	 * rs_times pointers were set up above.
 	 */
 	*rs_state = vx->current_runstate;
-	rs_times[0] = vx->runstate_entry_time | XEN_RUNSTATE_UPDATE;
+	rs_times[0] = entry_time;
 	memcpy(rs_times + 1, vx->runstate_times, sizeof(vx->runstate_times));
 
 	/* For the split case, we have to then copy it to the guest. */
@@ -396,8 +404,11 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 	smp_wmb();
 
 	/* Finally, clear the XEN_RUNSTATE_UPDATE bit. */
-	*update_bit = vx->runstate_entry_time >> 56;
+	if (update_bit) {
+		entry_time &= ~XEN_RUNSTATE_UPDATE;
+		*update_bit = entry_time >> 56;
 		smp_wmb();
+	}
 
 	if (user_len2)
 		read_unlock(&gpc2->lock);
@@ -619,6 +630,17 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		r = 0;
 		break;
 
+	case KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG:
+		if (!sched_info_on()) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		mutex_lock(&kvm->lock);
+		kvm->arch.xen.runstate_update_flag = !!data->u.runstate_update_flag;
+		mutex_unlock(&kvm->lock);
+		r = 0;
+		break;
+
 	default:
 		break;
 	}
@@ -656,6 +678,15 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		r = 0;
 		break;
 
+	case KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG:
+		if (!sched_info_on()) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		data->u.runstate_update_flag = kvm->arch.xen.runstate_update_flag;
+		r = 0;
+		break;
+
 	default:
 		break;
 	}

include/uapi/linux/kvm.h

@@ -1271,6 +1271,7 @@ struct kvm_x86_mce {
 #define KVM_XEN_HVM_CONFIG_RUNSTATE		(1 << 3)
 #define KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL	(1 << 4)
 #define KVM_XEN_HVM_CONFIG_EVTCHN_SEND		(1 << 5)
+#define KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG	(1 << 6)
 
 struct kvm_xen_hvm_config {
 	__u32 flags;
@@ -1776,6 +1777,7 @@ struct kvm_xen_hvm_attr {
 	union {
 		__u8 long_mode;
 		__u8 vector;
+		__u8 runstate_update_flag;
 		struct {
 			__u64 gfn;
 		} shared_info;
@@ -1816,6 +1818,8 @@ struct kvm_xen_hvm_attr {
 /* Available with KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_EVTCHN_SEND */
 #define KVM_XEN_ATTR_TYPE_EVTCHN		0x3
 #define KVM_XEN_ATTR_TYPE_XEN_VERSION		0x4
+/* Available with KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG */
+#define KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG	0x5
 
 /* Per-vCPU Xen attributes */
 #define KVM_XEN_VCPU_GET_ATTR	_IOWR(KVMIO, 0xca, struct kvm_xen_vcpu_attr)

tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c

@@ -440,6 +440,7 @@ int main(int argc, char *argv[])
 	TEST_REQUIRE(xen_caps & KVM_XEN_HVM_CONFIG_SHARED_INFO);
 
 	bool do_runstate_tests = !!(xen_caps & KVM_XEN_HVM_CONFIG_RUNSTATE);
+	bool do_runstate_flag = !!(xen_caps & KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG);
 	bool do_eventfd_tests = !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL);
 	bool do_evtchn_tests = do_eventfd_tests && !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_SEND);
 
@@ -475,6 +476,19 @@ int main(int argc, char *argv[])
 	};
 	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &lm);
 
+	if (do_runstate_flag) {
+		struct kvm_xen_hvm_attr ruf = {
+			.type = KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG,
+			.u.runstate_update_flag = 1,
+		};
+		vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &ruf);
+
+		ruf.u.runstate_update_flag = 0;
+		vm_ioctl(vm, KVM_XEN_HVM_GET_ATTR, &ruf);
+		TEST_ASSERT(ruf.u.runstate_update_flag == 1,
+			    "Failed to read back RUNSTATE_UPDATE_FLAG attr");
+	}
+
 	struct kvm_xen_hvm_attr ha = {
 		.type = KVM_XEN_ATTR_TYPE_SHARED_INFO,
 		.u.shared_info.gfn = SHINFO_REGION_GPA / PAGE_SIZE,