[PATCH v5 06/10] Docs: KVM: Add doc for the bitmap firmware registers

[Raghavendra Rao Ananta]

Add the documentation for the bitmap firmware registers in
hypercalls.rst and api.rst. This includes the details for
KVM_REG_ARM_STD_BMAP, KVM_REG_ARM_STD_HYP_BMAP, and
KVM_REG_ARM_VENDOR_HYP_BMAP registers.

Since the document is growing to carry other hypercall related
information, make necessary adjustments to present the document
in a generic sense, rather than being PSCI focused.

Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
---
 Documentation/virt/kvm/api.rst            | 17 ++++
 Documentation/virt/kvm/arm/hypercalls.rst | 95 ++++++++++++++++++-----
 2 files changed, 94 insertions(+), 18 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index d13fa6600467..e0107b157965 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -2542,6 +2542,23 @@ arm64 firmware pseudo-registers have the following bit pattern::
 
   0x6030 0000 0014 <regno:16>
 
+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.
+If there's no change in the value that's being written, 0 (success) is returned.
+
+(See Documentation/virt/kvm/arm/hypercalls.rst for more details.)
+
 arm64 SVE registers have the following bit patterns::
 
   0x6080 0000 0015 00 <n:5> <slice:5>   Zn bits[2048*slice + 2047 : 2048*slice]
diff --git a/Documentation/virt/kvm/arm/hypercalls.rst b/Documentation/virt/kvm/arm/hypercalls.rst
index d52c2e83b5b8..ccda9fc2d253 100644
--- a/Documentation/virt/kvm/arm/hypercalls.rst
+++ b/Documentation/virt/kvm/arm/hypercalls.rst
@@ -1,32 +1,32 @@
 .. 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
@@ -74,4 +74,63 @@ The following register is defined:
     KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_REQUIRED:
       The workaround is always active on this vCPU or it is not needed.
 
-.. [1] https://developer.arm.com/-/media/developer/pdf/ARM_DEN_0070A_Firmware_interfaces_for_mitigating_CVE-2017-5715.pdf
+
+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 electable 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 would't allow the userspace to configure the registers anymore once
+any of the vCPUs has run at least once. Instead, it will return a -EBUSY. However,
+if there's no change in the incoming value, it simply returns a success.
+
+The psuedo-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
+      function-id
+
+    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
\ No newline at end of file
-- 
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