KVM: x86: Add a framework for supporting MSR-based features

Provide a new KVM capability that allows bits within MSRs to be recognized
as features.  Two new ioctls are added to the /dev/kvm ioctl routine to
retrieve the list of these MSRs and then retrieve their values. A kvm_x86_ops
callback is used to determine support for the listed MSR-based features.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[Tweaked documentation. - Radim]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -123,14 +123,15 @@ memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
 flag KVM_VM_MIPS_VZ.
 
 
-4.3 KVM_GET_MSR_INDEX_LIST
+4.3 KVM_GET_MSR_INDEX_LIST, KVM_GET_MSR_FEATURE_INDEX_LIST
 
-Capability: basic
+Capability: basic, KVM_CAP_GET_MSR_FEATURES for KVM_GET_MSR_FEATURE_INDEX_LIST
 Architectures: x86
-Type: system
+Type: system ioctl
 Parameters: struct kvm_msr_list (in/out)
 Returns: 0 on success; -1 on error
 Errors:
+  EFAULT:    the msr index list cannot be read from or written to
   E2BIG:     the msr index list is to be to fit in the array specified by
              the user.
 
@@ -139,16 +140,23 @@ struct kvm_msr_list {
 	__u32 indices[0];
 };
 
-This ioctl returns the guest msrs that are supported.  The list varies
-by kvm version and host processor, but does not change otherwise.  The
-user fills in the size of the indices array in nmsrs, and in return
-kvm adjusts nmsrs to reflect the actual number of msrs and fills in
-the indices array with their numbers.
+The user fills in the size of the indices array in nmsrs, and in return
+kvm adjusts nmsrs to reflect the actual number of msrs and fills in the
+indices array with their numbers.
+
+KVM_GET_MSR_INDEX_LIST returns the guest msrs that are supported.  The list
+varies by kvm version and host processor, but does not change otherwise.
 
 Note: if kvm indicates supports MCE (KVM_CAP_MCE), then the MCE bank MSRs are
 not returned in the MSR list, as different vcpus can have a different number
 of banks, as set via the KVM_X86_SETUP_MCE ioctl.
 
+KVM_GET_MSR_FEATURE_INDEX_LIST returns the list of MSRs that can be passed
+to the KVM_GET_MSRS system ioctl.  This lets userspace probe host capabilities
+and processor features that are exposed via MSRs (e.g., VMX capabilities).
+This list also varies by kvm version and host processor, but does not change
+otherwise.
+
 
 4.4 KVM_CHECK_EXTENSION
 
@@ -475,14 +483,22 @@ Support for this has been removed.  Use KVM_SET_GUEST_DEBUG instead.
 
 4.18 KVM_GET_MSRS
 
-Capability: basic
+Capability: basic (vcpu), KVM_CAP_GET_MSR_FEATURES (system)
 Architectures: x86
-Type: vcpu ioctl
+Type: system ioctl, vcpu ioctl
 Parameters: struct kvm_msrs (in/out)
-Returns: 0 on success, -1 on error
+Returns: number of msrs successfully returned;
+        -1 on error
+
+When used as a system ioctl:
+Reads the values of MSR-based features that are available for the VM.  This
+is similar to KVM_GET_SUPPORTED_CPUID, but it returns MSR indices and values.
+The list of msr-based features can be obtained using KVM_GET_MSR_FEATURE_INDEX_LIST
+in a system ioctl.
 
+When used as a vcpu ioctl:
 Reads model-specific registers from the vcpu.  Supported msr indices can
-be obtained using KVM_GET_MSR_INDEX_LIST.
+be obtained using KVM_GET_MSR_INDEX_LIST in a system ioctl.
 
 struct kvm_msrs {
 	__u32 nmsrs; /* number of msrs in entries */

arch/x86/include/asm/kvm_host.h

@@ -1095,6 +1095,8 @@ struct kvm_x86_ops {
 	int (*mem_enc_op)(struct kvm *kvm, void __user *argp);
 	int (*mem_enc_reg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
 	int (*mem_enc_unreg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
+
+	int (*get_msr_feature)(struct kvm_msr_entry *entry);
 };
 
 struct kvm_arch_async_pf {

arch/x86/kvm/svm.c

@@ -3869,6 +3869,11 @@ static int cr8_write_interception(struct vcpu_svm *svm)
 	return 0;
 }
 
+static int svm_get_msr_feature(struct kvm_msr_entry *msr)
+{
+	return 1;
+}
+
 static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -6832,6 +6837,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
 	.vcpu_unblocking = svm_vcpu_unblocking,
 
 	.update_bp_intercept = update_bp_intercept,
+	.get_msr_feature = svm_get_msr_feature,
 	.get_msr = svm_get_msr,
 	.set_msr = svm_set_msr,
 	.get_segment_base = svm_get_segment_base,

arch/x86/kvm/vmx.c

@@ -3226,6 +3226,11 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
 	return !(val & ~valid_bits);
 }
 
+static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
+{
+	return 1;
+}
+
 /*
  * Reads an msr value (of 'msr_index') into 'pdata'.
  * Returns 0 on success, non-0 otherwise.
@@ -12296,6 +12301,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
 	.vcpu_put = vmx_vcpu_put,
 
 	.update_bp_intercept = update_exception_bitmap,
+	.get_msr_feature = vmx_get_msr_feature,
 	.get_msr = vmx_get_msr,
 	.set_msr = vmx_set_msr,
 	.get_segment_base = vmx_get_segment_base,

arch/x86/kvm/x86.c

@@ -1049,6 +1049,28 @@ static u32 emulated_msrs[] = {
 
 static unsigned num_emulated_msrs;
 
+/*
+ * List of msr numbers which are used to expose MSR-based features that
+ * can be used by a hypervisor to validate requested CPU features.
+ */
+static u32 msr_based_features[] = {
+};
+
+static unsigned int num_msr_based_features;
+
+static int do_get_msr_feature(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
+{
+	struct kvm_msr_entry msr;
+
+	msr.index = index;
+	if (kvm_x86_ops->get_msr_feature(&msr))
+		return 1;
+
+	*data = msr.data;
+
+	return 0;
+}
+
 bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
 {
 	if (efer & efer_reserved_bits)
@@ -2680,13 +2702,11 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
 		    int (*do_msr)(struct kvm_vcpu *vcpu,
 				  unsigned index, u64 *data))
 {
-	int i, idx;
+	int i;
 
-	idx = srcu_read_lock(&vcpu->kvm->srcu);
 	for (i = 0; i < msrs->nmsrs; ++i)
 		if (do_msr(vcpu, entries[i].index, &entries[i].data))
 			break;
-	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
 	return i;
 }
@@ -2785,6 +2805,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SET_BOOT_CPU_ID:
  	case KVM_CAP_SPLIT_IRQCHIP:
 	case KVM_CAP_IMMEDIATE_EXIT:
+	case KVM_CAP_GET_MSR_FEATURES:
 		r = 1;
 		break;
 	case KVM_CAP_ADJUST_CLOCK:
@@ -2899,6 +2920,31 @@ long kvm_arch_dev_ioctl(struct file *filp,
 			goto out;
 		r = 0;
 		break;
+	case KVM_GET_MSR_FEATURE_INDEX_LIST: {
+		struct kvm_msr_list __user *user_msr_list = argp;
+		struct kvm_msr_list msr_list;
+		unsigned int n;
+
+		r = -EFAULT;
+		if (copy_from_user(&msr_list, user_msr_list, sizeof(msr_list)))
+			goto out;
+		n = msr_list.nmsrs;
+		msr_list.nmsrs = num_msr_based_features;
+		if (copy_to_user(user_msr_list, &msr_list, sizeof(msr_list)))
+			goto out;
+		r = -E2BIG;
+		if (n < msr_list.nmsrs)
+			goto out;
+		r = -EFAULT;
+		if (copy_to_user(user_msr_list->indices, &msr_based_features,
+				 num_msr_based_features * sizeof(u32)))
+			goto out;
+		r = 0;
+		break;
+	}
+	case KVM_GET_MSRS:
+		r = msr_io(NULL, argp, do_get_msr_feature, 1);
+		break;
 	}
 	default:
 		r = -EINVAL;
@@ -3636,12 +3682,18 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		r = 0;
 		break;
 	}
-	case KVM_GET_MSRS:
+	case KVM_GET_MSRS: {
+		int idx = srcu_read_lock(&vcpu->kvm->srcu);
 		r = msr_io(vcpu, argp, do_get_msr, 1);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
-	case KVM_SET_MSRS:
+	}
+	case KVM_SET_MSRS: {
+		int idx = srcu_read_lock(&vcpu->kvm->srcu);
 		r = msr_io(vcpu, argp, do_set_msr, 0);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
+	}
 	case KVM_TPR_ACCESS_REPORTING: {
 		struct kvm_tpr_access_ctl tac;
 
@@ -4464,6 +4516,19 @@ static void kvm_init_msr_list(void)
 		j++;
 	}
 	num_emulated_msrs = j;
+
+	for (i = j = 0; i < ARRAY_SIZE(msr_based_features); i++) {
+		struct kvm_msr_entry msr;
+
+		msr.index = msr_based_features[i];
+		if (kvm_x86_ops->get_msr_feature(&msr))
+			continue;
+
+		if (j < i)
+			msr_based_features[j] = msr_based_features[i];
+		j++;
+	}
+	num_msr_based_features = j;
 }
 
 static int vcpu_mmio_write(struct kvm_vcpu *vcpu, gpa_t addr, int len,

include/uapi/linux/kvm.h

@@ -761,6 +761,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_TRACE_PAUSE           __KVM_DEPRECATED_MAIN_0x07
 #define KVM_TRACE_DISABLE         __KVM_DEPRECATED_MAIN_0x08
 #define KVM_GET_EMULATED_CPUID	  _IOWR(KVMIO, 0x09, struct kvm_cpuid2)
+#define KVM_GET_MSR_FEATURE_INDEX_LIST    _IOWR(KVMIO, 0x0a, struct kvm_msr_list)
 
 /*
  * Extension capability list.
@@ -934,6 +935,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_AIS_MIGRATION 150
 #define KVM_CAP_PPC_GET_CPU_CHAR 151
 #define KVM_CAP_S390_BPB 152
+#define KVM_CAP_GET_MSR_FEATURES 153
 
 #ifdef KVM_CAP_IRQ_ROUTING