Re: [RFC 1/7] x86/iommu: make AMD-Vi and Intel VT-d support configurable

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 20/12/2022 8:57 pm, Xenia Ragiadakou wrote:
>
> On 12/20/22 19:01, Jan Beulich wrote:
>> On 19.12.2022 07:34, Xenia Ragiadakou wrote:
>>> Currently, for x86 platforms, Xen does not provide to the users any
>>> configuration control over the IOMMU support and can only be built with
>>> both AMD and Intel IOMMU drivers enabled.
>>> However, there are use cases, e.g in safety-critical systems, that
>>> require
>>> Xen to be able to be configured to exclude unused code. A smaller
>>> tailored
>>> configuration would help Xen to meet faster certification
>>> requirements for
>>> individual platforms.
>>>
>>> Introduce two new Kconfig options, AMD_IOMMU and INTEL_VTD, to allow
>>> code
>>> specific to each IOMMU technology to be separated and, when not
>>> required,
>>> stripped. AMD_IOMMU enables IOMMU support for platforms that
>>> implement the
>>> AMD I/O Virtualization Technology. INTEL_VTD enables IOMMU support for
>>> platforms that implement the Intel Virtualization Technology for
>>> Directed I/O.
>>>
>>> Since no functional change is intended regarding the default
>>> configuration
>>> of an x86 system, both options depend on x86 and default to 'y'.
>>
>> But do things also build successfully when one or both options are
>> disabled?
>> I have to say that I would be quite surprised if that worked without
>> further
>> adjustments. In which case initially these options want to be
>> prompt-less,
>> with the prompts only added once 'n' also works.
>
> Without applying the whole series, disabling any of them or both won't
> work. Ok.

To do a multi-step implementation, you start with

config FOO
    bool y

then rearrange them main code to use CONFIG_FOO as appropriate, then
have a final patch that adds a Kconfig name, help text, etc which is
what makes the config option user selectable and able to be turned off.

~Andrew