On Mon, Aug 07 2023, Jing Zhang <jingzhangos@xxxxxxxxxx> wrote:
> Add some basic documentation on how to get feature ID register writable
> masks from userspace.
>
> Signed-off-by: Jing Zhang <jingzhangos@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index c0ddd3035462..92a9b20f970e 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6068,6 +6068,35 @@ writes to the CNTVCT_EL0 and CNTPCT_EL0 registers using the SET_ONE_REG
> interface. No error will be returned, but the resulting offset will not be
> applied.
>
> +4.139 KVM_ARM_GET_REG_WRITABLE_MASKS
> +-------------------------------------------
> +
> +:Capability: none
> +:Architectures: arm64
> +:Type: vm ioctl
> +:Parameters: struct reg_mask_range (in/out)
> +:Returns: 0 on success, < 0 on error
> +
> +
> +::
> +
> + #define ARM64_FEATURE_ID_SPACE_SIZE (3 * 8 * 8)
> +
> + struct reg_mask_range {
> + __u64 addr; /* Pointer to mask array */
> + __u64 reserved[7];
> + };
> +
> +This ioctl would copy the writable masks for feature ID registers to userspace.
> +The Feature ID space is defined as the System register space in AArch64 with
> +op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
> +To get the index in the mask array pointed by ``addr`` for a specified feature
> +ID register, use the macro ``ARM64_FEATURE_ID_SPACE_IDX(op0, op1, crn, crm, op2)``.
> +This allows the userspace to know upfront whether it can actually tweak the
> +contents of a feature ID register or not.
> +The ``reserved[7]`` is reserved for future use to add other register space. For
> +feature ID registers, it should be 0, otherwise, KVM may return error.
In case of future extensions, this means that userspace needs to figure
out what the kernel supports via different content in reg_mask_range
(i.e. try with a value in one of the currently reserved fields and fall
back to using addr only if that doesn't work.) Can we do better?
Maybe we could introduce a capability that returns the supported ranges
as flags, i.e. now we would return 1 for id regs masks, and for the
future case where we have some values in the next reserved field we
could return 1 & 2 etc. Would make life easier for userspace that needs
to work with different kernels, but might be overkill if reserved[] is
more like an insurance without any concrete plans for extensions.
