[RFC PATCH 1/2] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Yang Weijiang]

Enable KVM_{G,S}ET_ONE_REG uAPIs so that userspace can access HW MSR or
KVM synthetic MSR throught it.

In CET KVM series [*], KVM "steals" an MSR from PV MSR space and access
it via KVM_{G,S}ET_MSRs uAPIs, but the approach pollutes PV MSR space
and hides the difference of synthetic MSRs and normal HW defined MSRs.

Now carve out a separate room in KVM-customized MSR address space for
synthetic MSRs. The synthetic MSRs are not exposed to userspace via
KVM_GET_MSR_INDEX_LIST, instead userspace complies with KVM's setup and
composes the uAPI params. KVM synthetic MSR indices start from 0 and
increase linearly. Userspace caller should tag MSR type correctly in
order to access intended HW or synthetic MSR.

[*]:
https://lore.kernel.org/all/20240219074733.122080-18-weijiang.yang@xxxxxxxxx/

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 arch/x86/include/uapi/asm/kvm.h | 10 ++++++
 arch/x86/kvm/x86.c              | 62 +++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index ef11aa4cab42..ca2a47a85fa1 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -410,6 +410,16 @@ struct kvm_xcrs {
 	__u64 padding[16];
 };
 
+#define KVM_X86_REG_MSR			(1 << 2)
+#define KVM_X86_REG_SYNTHETIC_MSR	(1 << 3)
+
+struct kvm_x86_reg_id {
+	__u32 index;
+	__u8 type;
+	__u8 rsvd;
+	__u16 rsvd16;
+};
+
 #define KVM_SYNC_X86_REGS      (1UL << 0)
 #define KVM_SYNC_X86_SREGS     (1UL << 1)
 #define KVM_SYNC_X86_EVENTS    (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 91478b769af0..d0054c52f24b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2244,6 +2244,31 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 	return kvm_set_msr_ignored_check(vcpu, index, *data, true);
 }
 
+static int kvm_get_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *value)
+{
+	u64 val;
+	int r;
+
+	r = do_get_msr(vcpu, msr, &val);
+	if (r)
+		return r;
+
+	if (put_user(val, value))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int kvm_set_one_msr(struct kvm_vcpu *vcpu, u32 msr, u64 __user *value)
+{
+	u64 val;
+
+	if (get_user(val, value))
+		return -EFAULT;
+
+	return do_set_msr(vcpu, msr, &val);
+}
+
 #ifdef CONFIG_X86_64
 struct pvclock_clock {
 	int vclock_mode;
@@ -5859,6 +5884,11 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 	}
 }
 
+static int kvm_translate_synthetic_msr(u32 *index)
+{
+	return 0;
+}
+
 long kvm_arch_vcpu_ioctl(struct file *filp,
 			 unsigned int ioctl, unsigned long arg)
 {
@@ -5976,6 +6006,38 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
 	}
+	case KVM_GET_ONE_REG:
+	case KVM_SET_ONE_REG: {
+		struct kvm_x86_reg_id *id;
+		struct kvm_one_reg reg;
+		u64 __user *value;
+
+		r = -EFAULT;
+		if (copy_from_user(&reg, argp, sizeof(reg)))
+			break;
+
+		r = -EINVAL;
+		id = (struct kvm_x86_reg_id *)&reg.id;
+		if (id->rsvd || id->rsvd16)
+			break;
+
+		if (id->type != KVM_X86_REG_MSR &&
+		    id->type != KVM_X86_REG_SYNTHETIC_MSR)
+			break;
+
+		if (id->type == KVM_X86_REG_SYNTHETIC_MSR) {
+			r = kvm_translate_synthetic_msr(&id->index);
+			if (r)
+				break;
+		}
+
+		value = u64_to_user_ptr(reg.addr);
+		if (ioctl == KVM_GET_ONE_REG)
+			r = kvm_get_one_msr(vcpu, id->index, value);
+		else
+			r = kvm_set_one_msr(vcpu, id->index, value);
+		break;
+	}
 	case KVM_TPR_ACCESS_REPORTING: {
 		struct kvm_tpr_access_ctl tac;
 
-- 
2.43.0