KVM: x86: add system attribute to retrieve full set of supported xsave states

Because KVM_GET_SUPPORTED_CPUID is meant to be passed (by simple-minded
VMMs) to KVM_SET_CPUID2, it cannot include any dynamic xsave states that
have not been enabled.  Probing those, for example so that they can be
passed to ARCH_REQ_XCOMP_GUEST_PERM, requires a new ioctl or arch_prctl.
The latter is in fact worse, even though that is what the rest of the
API uses, because it would require supported_xcr0 to be moved from the
KVM module to the kernel just for this use.  In addition, the value
would be nonsensical (or an error would have to be returned) until
the KVM module is loaded in.

Therefore, to limit the growth of system ioctls, add a /dev/kvm
variant of KVM_{GET,HAS}_DEVICE_ATTR, and implement it in x86
with just one group (0) and attribute (KVM_X86_XCOMP_GUEST_SUPP).
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -3268,6 +3268,7 @@ number.
 
 :Capability: KVM_CAP_DEVICE_CTRL, KVM_CAP_VM_ATTRIBUTES for vm device,
              KVM_CAP_VCPU_ATTRIBUTES for vcpu device
+             KVM_CAP_SYS_ATTRIBUTES for system (/dev/kvm) device (no set)
 :Type: device ioctl, vm ioctl, vcpu ioctl
 :Parameters: struct kvm_device_attr
 :Returns: 0 on success, -1 on error
@@ -3303,6 +3304,7 @@ transferred is defined by the particular attribute.
 
 :Capability: KVM_CAP_DEVICE_CTRL, KVM_CAP_VM_ATTRIBUTES for vm device,
              KVM_CAP_VCPU_ATTRIBUTES for vcpu device
+             KVM_CAP_SYS_ATTRIBUTES for system (/dev/kvm) device
 :Type: device ioctl, vm ioctl, vcpu ioctl
 :Parameters: struct kvm_device_attr
 :Returns: 0 on success, -1 on error

arch/x86/include/uapi/asm/kvm.h

@@ -452,6 +452,9 @@ struct kvm_sync_regs {
 
 #define KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE	0x00000001
 
+/* attributes for system fd (group 0) */
+#define KVM_X86_XCOMP_GUEST_SUPP	0
+
 struct kvm_vmx_nested_state_data {
 	__u8 vmcs12[KVM_STATE_NESTED_VMX_VMCS_SIZE];
 	__u8 shadow_vmcs12[KVM_STATE_NESTED_VMX_VMCS_SIZE];

arch/x86/kvm/x86.c

@@ -4230,6 +4230,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SREGS2:
 	case KVM_CAP_EXIT_ON_EMULATION_FAILURE:
 	case KVM_CAP_VCPU_ATTRIBUTES:
+	case KVM_CAP_SYS_ATTRIBUTES:
 		r = 1;
 		break;
 	case KVM_CAP_EXIT_HYPERCALL:
@@ -4343,6 +4344,40 @@ static inline void __user *kvm_get_attr_addr(struct kvm_device_attr *attr)
 	return uaddr;
 }
 
+static int kvm_x86_dev_get_attr(struct kvm_device_attr *attr)
+{
+	u64 __user *uaddr = kvm_get_attr_addr(attr);
+
+	if (attr->group)
+		return -ENXIO;
+
+	if (IS_ERR(uaddr))
+		return PTR_ERR(uaddr);
+
+	switch (attr->attr) {
+	case KVM_X86_XCOMP_GUEST_SUPP:
+		if (put_user(supported_xcr0, uaddr))
+			return -EFAULT;
+		return 0;
+	default:
+		return -ENXIO;
+		break;
+	}
+}
+
+static int kvm_x86_dev_has_attr(struct kvm_device_attr *attr)
+{
+	if (attr->group)
+		return -ENXIO;
+
+	switch (attr->attr) {
+	case KVM_X86_XCOMP_GUEST_SUPP:
+		return 0;
+	default:
+		return -ENXIO;
+	}
+}
+
 long kvm_arch_dev_ioctl(struct file *filp,
 			unsigned int ioctl, unsigned long arg)
 {
@@ -4431,6 +4466,22 @@ long kvm_arch_dev_ioctl(struct file *filp,
 	case KVM_GET_SUPPORTED_HV_CPUID:
 		r = kvm_ioctl_get_supported_hv_cpuid(NULL, argp);
 		break;
+	case KVM_GET_DEVICE_ATTR: {
+		struct kvm_device_attr attr;
+		r = -EFAULT;
+		if (copy_from_user(&attr, (void __user *)arg, sizeof(attr)))
+			break;
+		r = kvm_x86_dev_get_attr(&attr);
+		break;
+	}
+	case KVM_HAS_DEVICE_ATTR: {
+		struct kvm_device_attr attr;
+		r = -EFAULT;
+		if (copy_from_user(&attr, (void __user *)arg, sizeof(attr)))
+			break;
+		r = kvm_x86_dev_has_attr(&attr);
+		break;
+	}
 	default:
 		r = -EINVAL;
 		break;

include/uapi/linux/kvm.h

@@ -1133,6 +1133,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
 #define KVM_CAP_VM_GPA_BITS 207
 #define KVM_CAP_XSAVE2 208
+#define KVM_CAP_SYS_ATTRIBUTES 209
 
 #ifdef KVM_CAP_IRQ_ROUTING