[PATCH v9 01/11] KVM: arm64: Allow userspace to get the writable masks for feature ID registers

[Jing Zhang <jingzhangos@google.com>]

Add a VM ioctl to allow userspace to get writable masks for feature ID
registers in below system register space:
op0 = 3, op1 = {0, 1, 3}, CRn = 0, CRm = {0 - 7}, op2 = {0 - 7}
This is used to support mix-and-match userspace and kernels for writable
ID registers, where userspace may want to know upfront whether it can
actually tweak the contents of an idreg or not.

Suggested-by: Marc Zyngier <maz@kernel.org>
Suggested-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 arch/arm64/include/asm/kvm_host.h |  2 ++
 arch/arm64/include/uapi/asm/kvm.h | 26 ++++++++++++++
 arch/arm64/kvm/arm.c              |  7 ++++
 arch/arm64/kvm/sys_regs.c         | 57 +++++++++++++++++++++++++++++++
 include/uapi/linux/kvm.h          |  1 +
 5 files changed, 93 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index d3dd05bbfe23..a328d362df5a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1074,6 +1074,8 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
 			       struct kvm_arm_copy_mte_tags *copy_tags);
 int kvm_vm_ioctl_set_counter_offset(struct kvm *kvm,
 				    struct kvm_arm_counter_offset *offset);
+int kvm_vm_ioctl_get_reg_writable_masks(struct kvm *kvm,
+					struct reg_mask_range *range);
 
 /* Guest/host FPSIMD coordination helpers */
 int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index f7ddd73a8c0f..7a21bbb8a0f7 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -505,6 +505,32 @@ struct kvm_smccc_filter {
 #define KVM_HYPERCALL_EXIT_SMC		(1U << 0)
 #define KVM_HYPERCALL_EXIT_16BIT	(1U << 1)
 
+/* Get feature ID registers userspace writable mask. */
+/*
+ * From DDI0487J.a, D19.2.66 ("ID_AA64MMFR2_EL1, AArch64 Memory Model
+ * Feature Register 2"):
+ *
+ * "The Feature ID space is defined as the System register space in
+ * AArch64 with op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7},
+ * op2=={0-7}."
+ *
+ * This covers all R/O registers that indicate anything useful feature
+ * wise, including the ID registers.
+ */
+#define ARM64_FEATURE_ID_SPACE_IDX(op0, op1, crn, crm, op2)		\
+	({								\
+		__u64 __op1 = (op1) & 3;				\
+		__op1 -= (__op1 == 3);					\
+		(__op1 << 6 | ((crm) & 7) << 3 | (op2));		\
+	})
+
+#define ARM64_FEATURE_ID_SPACE_SIZE	(3 * 8 * 8)
+
+struct reg_mask_range {
+	__u64 addr;		/* Pointer to mask array */
+	__u64 reserved[7];
+};
+
 #endif
 
 #endif /* __ARM_KVM_H__ */
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index d1cb298a58a0..cfb5e9ef43c4 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1630,6 +1630,13 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 
 		return kvm_vm_set_attr(kvm, &attr);
 	}
+	case KVM_ARM_GET_REG_WRITABLE_MASKS: {
+		struct reg_mask_range range;
+
+		if (copy_from_user(&range, argp, sizeof(range)))
+			return -EFAULT;
+		return kvm_vm_ioctl_get_reg_writable_masks(kvm, &range);
+	}
 	default:
 		return -EINVAL;
 	}
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 2ca2973abe66..216905840c92 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -3560,6 +3560,63 @@ int kvm_arm_copy_sys_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
 	return write_demux_regids(uindices);
 }
 
+#define ARM64_FEATURE_ID_SPACE_INDEX(r)			\
+	ARM64_FEATURE_ID_SPACE_IDX(sys_reg_Op0(r),	\
+		sys_reg_Op1(r),				\
+		sys_reg_CRn(r),				\
+		sys_reg_CRm(r),				\
+		sys_reg_Op2(r))
+
+static bool is_feature_id_reg(u32 encoding)
+{
+	return (sys_reg_Op0(encoding) == 3 &&
+		(sys_reg_Op1(encoding) < 2 || sys_reg_Op1(encoding) == 3) &&
+		sys_reg_CRn(encoding) == 0 &&
+		sys_reg_CRm(encoding) <= 7);
+}
+
+int kvm_vm_ioctl_get_reg_writable_masks(struct kvm *kvm, struct reg_mask_range *range)
+{
+	const void *zero_page = page_to_virt(ZERO_PAGE(0));
+	u64 __user *masks = (u64 __user *)range->addr;
+
+	/* Only feature id range is supported, reserved[7] must be zero. */
+	if (memcmp(range->reserved, zero_page, sizeof(range->reserved)))
+		return -EINVAL;
+
+	/* Wipe the whole thing first */
+	if (clear_user(masks, ARM64_FEATURE_ID_SPACE_SIZE * sizeof(__u64)))
+		return -EFAULT;
+
+	for (int i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
+		const struct sys_reg_desc *reg = &sys_reg_descs[i];
+		u32 encoding = reg_to_encoding(reg);
+		u64 val;
+
+		if (!is_feature_id_reg(encoding) || !reg->set_user)
+			continue;
+
+		/*
+		 * For ID registers, we return the writable mask. Other feature
+		 * registers return a full 64bit mask. That's not necessary
+		 * compliant with a given revision of the architecture, but the
+		 * RES0/RES1 definitions allow us to do that.
+		 */
+		if (is_id_reg(encoding)) {
+			if (!reg->val)
+				continue;
+			val = reg->val;
+		} else {
+			val = ~0UL;
+		}
+
+		if (put_user(val, (masks + ARM64_FEATURE_ID_SPACE_INDEX(encoding))))
+			return -EFAULT;
+	}
+
+	return 0;
+}
+
 int __init kvm_sys_reg_table_init(void)
 {
 	struct sys_reg_params params;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index f089ab290978..424b6d00440b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1555,6 +1555,7 @@ struct kvm_s390_ucas_mapping {
 #define KVM_ARM_MTE_COPY_TAGS	  _IOR(KVMIO,  0xb4, struct kvm_arm_copy_mte_tags)
 /* Available with KVM_CAP_COUNTER_OFFSET */
 #define KVM_ARM_SET_COUNTER_OFFSET _IOW(KVMIO,  0xb5, struct kvm_arm_counter_offset)
+#define KVM_ARM_GET_REG_WRITABLE_MASKS _IOR(KVMIO,  0xb6, struct reg_mask_range)
 
 /* ioctl for vm fd */
 #define KVM_CREATE_DEVICE	  _IOWR(KVMIO,  0xe0, struct kvm_create_device)
-- 
2.42.0.rc1.204.g551eb34607-goog


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