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

[Reiji Watanabe <reijiw@google.com>]

Hi Raghu,

On Tue, Jan 4, 2022 at 11:49 AM Raghavendra Rao Ananta
<rananta@google.com> wrote:
>
> 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 <rananta@google.com>
> ---
>  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

IMHO, instead of including its format of the register in the name,
including its purpose/function in the name might be better.
e.g. KVM_CAP_ARM_HVC_FEATURE_REG ?
(Feature fields don't necessarily have to be in a bitmap format
 if they don't fit well although I'm not sure if we have such fields.)

> +--------------------------------
> +
> +:Architectures: arm64
> +:Parameters: None
> +:Returns: Number of psuedo-firmware registers supported

Looking at patch-4, the return value of this would be the number of
pseudo-firmware *bitmap* registers supported.
BTW, "4.68 KVM_SET_ONE_REG" in the doc uses the word "arm64 firmware
pseudo-registers".  It would be nicer to use the same term.

> +
> +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.

Since even if KVM_RUN fails, and the VM hasn't started yet,
it will get immutable. So, "after any of the vCPUs run KVM_RUN."
might be more clear ?

Thanks,
Reiji



> +
>  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
>