After the vcpu's SVE configuration is finalized, further attempts to
write this register will fail with EPERM.
+arm64 bitmap feature firmware pseudo-registers have the following bit pattern::
+
+ 0x6030 0000 0016 <regno:16>
+
+The bitmap feature firmware registers exposes the hypercall services that
+are available for userspace to configure. The set bits corresponds to the
+services that are available for the guests to access. By default, KVM
+sets all the supported bits during VM initialization. The userspace can
+discover the available services via KVM_GET_ONE_REG, and write back the
+bitmap corresponding to the features that it wishes guests to see via
+KVM_SET_ONE_REG.
+
+Note: These registers are immutable once any of the vCPUs of the VM has
+run at least once. A KVM_SET_ONE_REG in such a scenario will return
+a -EBUSY to userspace.
+
+(See Documentation/virt/kvm/arm/hypercalls.rst for more details.)
+
MIPS registers are mapped using the lower 32 bits. The upper 16 of that is
the register group type:
.. SPDX-License-Identifier: GPL-2.0
-=========================================
-Power State Coordination Interface (PSCI)
-=========================================
+=======================
+ARM Hypercall Interface
+=======================
-KVM implements the PSCI (Power State Coordination Interface)
-specification in order to provide services such as CPU on/off, reset
-and power-off to the guest.
+KVM handles the hypercall services as requested by the guests. New hypercall
+services are regularly made available by the ARM specification or by KVM (as
+vendor services) if they make sense from a virtualization point of view.
-The PSCI specification is regularly updated to provide new features,
-and KVM implements these updates if they make sense from a virtualization
-point of view.
-
-This means that a guest booted on two different versions of KVM can
-observe two different "firmware" revisions. This could cause issues if
-a given guest is tied to a particular PSCI revision (unlikely), or if
-a migration causes a different PSCI version to be exposed out of the
-blue to an unsuspecting guest.
+This means that a guest booted on two different versions of KVM can observe
+two different "firmware" revisions. This could cause issues if a given guest
+is tied to a particular version of a hypercall service, or if a migration
+causes a different version to be exposed out of the blue to an unsuspecting
+guest.
In order to remedy this situation, KVM exposes a set of "firmware
pseudo-registers" that can be manipulated using the GET/SET_ONE_REG
interface. These registers can be saved/restored by userspace, and set
-to a convenient value if required.
+to a convenient value as required.
-The following register is defined:
+The following registers are defined:
* KVM_REG_ARM_PSCI_VERSION:
+ KVM implements the PSCI (Power State Coordination Interface)
+ specification in order to provide services such as CPU on/off, reset
+ and power-off to the guest.
+
- Only valid if the vcpu has the KVM_ARM_VCPU_PSCI_0_2 feature set
(and thus has already been initialized)
- Returns the current PSCI version on GET_ONE_REG (defaulting to the
KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_REQUIRED:
The workaround is always active on this vCPU or it is not needed.
+
+Bitmap Feature Firmware Registers
+---------------------------------
+
+Contrary to the above registers, the following registers exposes the
+hypercall services in the form of a feature-bitmap to the userspace. This
+bitmap is translated to the services that are available to the guest.
+There is a register defined per service call owner and can be accessed via
+GET/SET_ONE_REG interface.
+
+By default, these registers are set with the upper limit of the features
+that are supported. This way userspace can discover all the usable
+hypercall services via GET_ONE_REG. The user-space can write-back the
+desired bitmap back via SET_ONE_REG. The features for the registers that
+are untouched, probably because userspace isn't aware of them, will be
+exposed as is to the guest.
+
+Note that KVM will not allow the userspace to configure the registers
+anymore once any of the vCPUs has run at least once. Instead, it will
+return a -EBUSY.
+
+The pseudo-firmware bitmap register are as follows:
+
+* KVM_REG_ARM_STD_BMAP:
+ Controls the bitmap of the ARM Standard Secure Service Calls.
+
+ The following bits are accepted:
+
+ Bit-0: KVM_REG_ARM_STD_BIT_TRNG_V1_0:
+ The bit represents the services offered under v1.0 of ARM True Random
+ Number Generator (TRNG) specification, ARM DEN0098.
+
+* KVM_REG_ARM_STD_HYP_BMAP:
+ Controls the bitmap of the ARM Standard Hypervisor Service Calls.
+
+ The following bits are accepted:
+
+ Bit-0: KVM_REG_ARM_STD_HYP_BIT_PV_TIME:
+ The bit represents the Paravirtualized Time service as represented by
+ ARM DEN0057A.
+
+* KVM_REG_ARM_VENDOR_HYP_BMAP:
+ Controls the bitmap of the Vendor specific Hypervisor Service Calls.
+
+ The following bits are accepted:
+
+ Bit-0: KVM_REG_ARM_VENDOR_HYP_BIT_FUNC_FEAT
+ The bit represents the ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID
+ and ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID function-ids.
+
+ Bit-1: KVM_REG_ARM_VENDOR_HYP_BIT_PTP:
+ The bit represents the Precision Time Protocol KVM service.
+
+Errors:
+
+ ======= =============================================================
+ -ENOENT Unknown register accessed.
+ -EBUSY Attempt a 'write' to the register after the VM has started.
+ -EINVAL Invalid bitmap written to the register.
+ ======= =============================================================
+
.. [1] https://developer.arm.com/-/media/developer/pdf/ARM_DEN_0070A_Firmware_interfaces_for_mitigating_CVE-2017-5715.pdf
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