[PATCH v7 22/38] KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_CONFIGURABLE capability

[Reiji Watanabe]

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 at google.com>
---
 Documentation/virt/kvm/api.rst | 16 ++++++++++++++++
 arch/arm64/kvm/arm.c           |  1 +
 include/uapi/linux/kvm.h       |  1 +
 3 files changed, 18 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 85c7abc51af5..e2e7b08e64c1 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -2601,6 +2601,14 @@ EINVAL.
 After the vcpu's SVE configuration is finalized, further attempts to
 write this register will fail with EPERM.
 
+The arm64 ID registers with encoding Op0=3, Op1=0, CRn=0, 1<=CRm<8, 0<=Op2<8
+are allowed to modified by userspace only for AArch64 EL1 vCPUs 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).
+Those ID registers are always immutable for AArch32 EL1 vCPUs, which
+KVM_ARM_VCPU_EL1_32BIT is configured for, even when
+KVM_CAP_ARM_ID_REG_CONFIGURABLE is available.
 
 MIPS registers are mapped using the lower 32 bits.  The upper 16 of that is
 the register group type:
@@ -7724,6 +7732,14 @@ At this time, KVM_PMU_CAP_DISABLE is the only capability.  Setting
 this capability will disable PMU virtualization for that VM.  Usermode
 should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
 
+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.
+
 9. Known KVM API problems
 =========================
 
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 5c1cee04aa95..b4db368948cc 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -211,6 +211,7 @@ 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 91a6fe4e02c0..171f1d0ea1e1 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1144,6 +1144,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_MEM_OP_EXTENSION 211
 #define KVM_CAP_PMU_CAPABILITY 212
 #define KVM_CAP_DISABLE_QUIRKS2 213
+#define KVM_CAP_ARM_ID_REG_CONFIGURABLE 214
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.36.0.rc0.470.gd361397f0d-goog