Merge branch kvm-arm64/writable-id-regs into kvmarm/next

* kvm-arm64/writable-id-regs:
  : Writable ID registers, courtesy of Jing Zhang
  :
  : This series significantly expands the architectural feature set that
  : userspace can manipulate via the ID registers. A new ioctl is defined
  : that makes the mutable fields in the ID registers discoverable to
  : userspace.
  KVM: selftests: Avoid using forced target for generating arm64 headers
  tools headers arm64: Fix references to top srcdir in Makefile
  KVM: arm64: selftests: Test for setting ID register from usersapce
  tools headers arm64: Update sysreg.h with kernel sources
  KVM: selftests: Generate sysreg-defs.h and add to include path
  perf build: Generate arm64's sysreg-defs.h and add to include path
  tools: arm64: Add a Makefile for generating sysreg-defs.h
  KVM: arm64: Document vCPU feature selection UAPIs
  KVM: arm64: Allow userspace to change ID_AA64ZFR0_EL1
  KVM: arm64: Allow userspace to change ID_AA64PFR0_EL1
  KVM: arm64: Allow userspace to change ID_AA64MMFR{0-2}_EL1
  KVM: arm64: Allow userspace to change ID_AA64ISAR{0-2}_EL1
  KVM: arm64: Bump up the default KVM sanitised debug version to v8p8
  KVM: arm64: Reject attempts to set invalid debug arch version
  KVM: arm64: Advertise selected DebugVer in DBGDIDR.Version
  KVM: arm64: Use guest ID register values for the sake of emulation
  KVM: arm64: Document KVM_ARM_GET_REG_WRITABLE_MASKS
  KVM: arm64: Allow userspace to get the writable masks for feature ID registers

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

3 files changed, 101 insertions, 0 deletions

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 21a7578142a1..d7ce71e27c33 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3370,6 +3370,8 @@ return indicates the attribute is implemented.  It does not necessarily
 indicate that the attribute can be read or written in the device's
 current state.  "addr" is ignored.
 
+.. _KVM_ARM_VCPU_INIT:
+
 4.82 KVM_ARM_VCPU_INIT
 ----------------------
 
@@ -6070,6 +6072,56 @@ writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
 interface. No error will be returned, but the resulting offset will not be
 applied.
 
+.. _KVM_ARM_GET_REG_WRITABLE_MASKS:
+
+4.139 KVM_ARM_GET_REG_WRITABLE_MASKS
+-------------------------------------------
+
+:Capability: KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES
+:Architectures: arm64
+:Type: vm ioctl
+:Parameters: struct reg_mask_range (in/out)
+:Returns: 0 on success, < 0 on error
+
+
+::
+
+        #define KVM_ARM_FEATURE_ID_RANGE	0
+        #define KVM_ARM_FEATURE_ID_RANGE_SIZE	(3 * 8 * 8)
+
+        struct reg_mask_range {
+                __u64 addr;             /* Pointer to mask array */
+                __u32 range;            /* Requested range */
+                __u32 reserved[13];
+        };
+
+This ioctl copies the writable masks for a selected range of registers to
+userspace.
+
+The ``addr`` field is a pointer to the destination array where KVM copies
+the writable masks.
+
+The ``range`` field indicates the requested range of registers.
+``KVM_CHECK_EXTENSION`` for the ``KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES``
+capability returns the supported ranges, expressed as a set of flags. Each
+flag's bit index represents a possible value for the ``range`` field.
+All other values are reserved for future use and KVM may return an error.
+
+The ``reserved[13]`` array is reserved for future use and should be 0, or
+KVM may return an error.
+
+KVM_ARM_FEATURE_ID_RANGE (0)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Feature ID range is defined as the AArch64 System register space with
+op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
+
+The mask returned array pointed to by ``addr`` is indexed by the macro
+``ARM64_FEATURE_ID_RANGE_IDX(op0, op1, crn, crm, op2)``, allowing userspace
+to know what fields can be changed for the system register described by
+``op0, op1, crn, crm, op2``. KVM rejects ID register values that describe a
+superset of the features supported by the system.
+
 5. The kvm_run structure
 ========================
 
diff --git a/Documentation/virt/kvm/arm/index.rst b/Documentation/virt/kvm/arm/index.rst
index e84848432158..7f231c724e16 100644
--- a/Documentation/virt/kvm/arm/index.rst
+++ b/Documentation/virt/kvm/arm/index.rst
@@ -11,3 +11,4 @@ ARM
    hypercalls
    pvtime
    ptp_kvm
+   vcpu-features
diff --git a/Documentation/virt/kvm/arm/vcpu-features.rst b/Documentation/virt/kvm/arm/vcpu-features.rst
new file mode 100644
index 000000000000..f7cc6d8d8b74
--- /dev/null
+++ b/Documentation/virt/kvm/arm/vcpu-features.rst
@@ -0,0 +1,48 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===============================
+vCPU feature selection on arm64
+===============================
+
+KVM/arm64 provides two mechanisms that allow userspace to configure
+the CPU features presented to the guest.
+
+KVM_ARM_VCPU_INIT
+=================
+
+The ``KVM_ARM_VCPU_INIT`` ioctl accepts a bitmap of feature flags
+(``struct kvm_vcpu_init::features``). Features enabled by this interface are
+*opt-in* and may change/extend UAPI. See :ref:`KVM_ARM_VCPU_INIT` for complete
+documentation of the features controlled by the ioctl.
+
+Otherwise, all CPU features supported by KVM are described by the architected
+ID registers.
+
+The ID Registers
+================
+
+The Arm architecture specifies a range of *ID Registers* that describe the set
+of architectural features supported by the CPU implementation. KVM initializes
+the guest's ID registers to the maximum set of CPU features supported by the
+system. The ID register values may be VM-scoped in KVM, meaning that the
+values could be shared for all vCPUs in a VM.
+
+KVM allows userspace to *opt-out* of certain CPU features described by the ID
+registers by writing values to them via the ``KVM_SET_ONE_REG`` ioctl. The ID
+registers are mutable until the VM has started, i.e. userspace has called
+``KVM_RUN`` on at least one vCPU in the VM. Userspace can discover what fields
+are mutable in the ID registers using the ``KVM_ARM_GET_REG_WRITABLE_MASKS``.
+See the :ref:`ioctl documentation <KVM_ARM_GET_REG_WRITABLE_MASKS>` for more
+details.
+
+Userspace is allowed to *limit* or *mask* CPU features according to the rules
+outlined by the architecture in DDI0487J.a D19.1.3 'Principles of the ID
+scheme for fields in ID register'. KVM does not allow ID register values that
+exceed the capabilities of the system.
+
+.. warning::
+   It is **strongly recommended** that userspace modify the ID register values
+   before accessing the rest of the vCPU's CPU register state. KVM may use the
+   ID register values to control feature emulation. Interleaving ID register
+   modification with other system register accesses may lead to unpredictable
+   behavior.