[RFC PATCH v4 15/26] KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_CONFIGURABLE capability

[Reiji Watanabe <reijiw@google.com>]

Introduce a new capability KVM_CAP_ARM_ID_REG_CONFIGURABLE to indicate
that ID registers are writable by userspace.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
---
 Documentation/virt/kvm/api.rst | 12 ++++++++++++
 arch/arm64/kvm/arm.c           |  1 +
 include/uapi/linux/kvm.h       |  1 +
 3 files changed, 14 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aeeb071c7688..86099a9b0bae 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -2574,6 +2574,10 @@ EINVAL.
 After the vcpu's SVE configuration is finalized, further attempts to
 write this register will fail with EPERM.
 
+The arm64 ID registers (where Op0=3, Op1=0, CRn=0, 0<=CRm<8, 0<=Op2<8)
+are allowed to set by userspace if KVM_CAP_ARM_ID_REG_CONFIGURABLE is
+available.  They become immutable after calling KVM_RUN on any of the
+vcpus in the guest (modifying values of those registers will fail).
 
 MIPS registers are mapped using the lower 32 bits.  The upper 16 of that is
 the register group type:
@@ -7484,3 +7488,11 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
 of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
 the hypercalls whose corresponding bit is in the argument, and return
 ENOSYS for the others.
+
+8.35 KVM_CAP_ARM_ID_REG_CONFIGURABLE
+------------------------------------
+
+:Architectures: arm64
+
+This capability indicates that userspace can modify the ID registers
+via KVM_SET_ONE_REG ioctl.
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 16fc2ce32069..876f2777acf2 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -216,6 +216,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SET_GUEST_DEBUG:
 	case KVM_CAP_VCPU_ATTRIBUTES:
 	case KVM_CAP_PTP_KVM:
+	case KVM_CAP_ARM_ID_REG_CONFIGURABLE:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 1daa45268de2..9697c06a7f5b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1131,6 +1131,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
 #define KVM_CAP_ARM_MTE 205
 #define KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM 206
+#define KVM_CAP_ARM_ID_REG_CONFIGURABLE 207
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.34.1.448.ga2b2bfdf31-goog

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