KVM: x86/xen: Kernel acceleration for XENVER_version

Turns out this is a fast path for PV guests because they use it to
trigger the event channel upcall. So letting it bounce all the way up
to userspace is not great.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-14-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1026,6 +1026,7 @@ struct msr_bitmap_range {
 
 /* Xen emulation context */
 struct kvm_xen {
+	u32 xen_version;
 	bool long_mode;
 	u8 upcall_vector;
 	struct gfn_to_pfn_cache shinfo_cache;

arch/x86/kvm/xen.c

@@ -18,6 +18,7 @@
 #include <trace/events/kvm.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/vcpu.h>
+#include <xen/interface/version.h>
 #include <xen/interface/event_channel.h>
 #include <xen/interface/sched.h>
 
@@ -465,6 +466,13 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		r = kvm_xen_setattr_evtchn(kvm, data);
 		break;
 
+	case KVM_XEN_ATTR_TYPE_XEN_VERSION:
+		mutex_lock(&kvm->lock);
+		kvm->arch.xen.xen_version = data->u.xen_version;
+		mutex_unlock(&kvm->lock);
+		r = 0;
+		break;
+
 	default:
 		break;
 	}
@@ -497,6 +505,11 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		r = 0;
 		break;
 
+	case KVM_XEN_ATTR_TYPE_XEN_VERSION:
+		data->u.xen_version = kvm->arch.xen.xen_version;
+		r = 0;
+		break;
+
 	default:
 		break;
 	}
@@ -1059,6 +1072,12 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 				params[3], params[4], params[5]);
 
 	switch (input) {
+	case __HYPERVISOR_xen_version:
+		if (params[0] == XENVER_version && vcpu->kvm->arch.xen.xen_version) {
+			r = vcpu->kvm->arch.xen.xen_version;
+			handled = true;
+		}
+		break;
 	case __HYPERVISOR_event_channel_op:
 		if (params[0] == EVTCHNOP_send)
 			handled = kvm_xen_hcall_evtchn_send(vcpu, params[1], &r);

include/uapi/linux/kvm.h

@@ -1711,7 +1711,7 @@ struct kvm_xen_hvm_attr {
 				__u32 padding[4];
 			} deliver;
 		} evtchn;
-
+		__u32 xen_version;
 		__u64 pad[8];
 	} u;
 };
@@ -1722,6 +1722,7 @@ struct kvm_xen_hvm_attr {
 #define KVM_XEN_ATTR_TYPE_UPCALL_VECTOR		0x2
 /* 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
 
 /* Per-vCPU Xen attributes */
 #define KVM_XEN_VCPU_GET_ATTR	_IOWR(KVMIO, 0xca, struct kvm_xen_vcpu_attr)