Re: [RFC PATCH v2 18/28] KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_WRITABLE capability

[Oliver Upton <oupton@google.com>]

On Tue, Nov 02, 2021 at 11:25:10PM -0700, Reiji Watanabe wrote:
> Introduce a new capability KVM_CAP_ARM_ID_REG_WRITABLE to indicate
> that ID registers are writable by userspace.
> 
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 8 ++++++++
>  arch/arm64/kvm/arm.c           | 1 +
>  include/uapi/linux/kvm.h       | 1 +
>  3 files changed, 10 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a6729c8cf063..f7dfb5127310 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7265,3 +7265,11 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
>  of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
>  the hypercalls whose corresponding bit is in the argument, and return
>  ENOSYS for the others.
> +
> +8.35 KVM_CAP_ARM_ID_REG_WRITABLE
> +--------------------------------

ID registers are technically already writable, KVM just rejects any
value other than what it derives from sanitising the host ID registers.
I agree that the nuance being added warrants a KVM_CAP, as it informs
userspace it can deliberately configure ID registers with a more limited
value than what KVM returns.

KVM_CAP_ARM_ID_REG_CONFIGURABLE maybe? Naming is hard :)

--
Thanks,
Oliver
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