Subject: [RFC PATCH v3 03/11] KVM: Introduce KVM_CAP_ARM_HVC_FW_REG_BMAP

From: Raghavendra Rao Ananta <[email-masked]>
Date: Tue, 4 Jan 2022 19:49:10 +0000

Introduce the KVM ARM64 capability, KVM_CAP_ARM_HVC_FW_REG_BMAP,
to indicate the support for psuedo-firmware bitmap extension.
Each of these registers holds a feature-set exposed to the guest
in the form of a bitmap. If supported, a simple 'read' of the
capability should return the number of psuedo-firmware registers
supported. User-space can utilize this to discover the registers.
It can further explore or modify the features using the classical
GET/SET_ONE_REG interface.

Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
---
 Documentation/virt/kvm/api.rst | 21 +++++++++++++++++++++
 include/uapi/linux/kvm.h       |  1 +
 2 files changed, 22 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aeeb071c7688..646176537f2c 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6925,6 +6925,27 @@ indicated by the fd to the VM this is called on.
 This is intended to support intra-host migration of VMs between userspace VMMs,
 upgrading the VMM process without interrupting the guest.
 
+7.30 KVM_CAP_ARM_HVC_FW_REG_BMAP
+--------------------------------
+
+:Architectures: arm64
+:Parameters: None
+:Returns: Number of psuedo-firmware registers supported
+
+This capability indicates that KVM for arm64 supports the psuedo-firmware
+register bitmap extension. Each of these registers represent the features
+supported by a particular type in the form of a bitmap. By default, these
+registers are set with the upper limit of the features that are supported.
+
+The registers can be accessed via the standard SET_ONE_REG and KVM_GET_ONE_REG
+interfaces. The user-space is expected to read the number of these registers
+available by reading KVM_CAP_ARM_HVC_FW_REG_BMAP, read the current bitmap
+configuration via GET_ONE_REG for each register, and then write back the
+desired bitmap of features that it wishes the guest to see via SET_ONE_REG.
+
+Note that KVM doesn't allow the user-space to modify these registers after
+the VM (any of the vCPUs) has started running.
+
 8. Other capabilities.
 ======================
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 1daa45268de2..209b43dbbc3c 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_HVC_FW_REG_BMAP 207
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.34.1.448.ga2b2bfdf31-goog

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