Re: [PATCH RFC 13/20] Documentation: ctlv: Describe how to write a family

[Jiri Pirko]

Wed, Oct 21, 2026 at 06:48:57PM +0200, jgg@xxxxxxxxxx wrote:
>On Wed, Oct 21, 2026 at 06:24:12PM +0200, Jiri Pirko wrote:
>> >I haven't got that far in the patches, but I think it would be good to
>> >spend more time here explaining the fundamentals of how the extension
>> >mechanism is expected to be used..
>> >
>> >It looks like RDMA to me where you have some 'common' and some
>> >'driver' parts but at this point in the docs it isn't clear how that
>> >is reached. Like does each driver gets its own family? What is a
>> 
>> Depends on the consumer/subsystem. But esentially yes. That is what I'm
>> doing for "attestation". This patchset introduces ctlv-attestation-tdx.
>> Next we will have ctlv-attestation-tpm, ctlv-attestation-firme,
>> ctlv-attestation-armcca ctlv-attestation-spdm and others.
>
>Hmm one of the things iommufd does is to have several different
>drivers active at once within the same FD. There is only one char dev
>for the whole system and the VMM would operate all the hardware
>through it.
>
>So in that model individual calls are marked with the driver's
>identity when any driver specific data is passed.
>
>I guess that doesn't quite work in this model with multiple-families?

Nope.

>
>But it does work well here, and matches what rdma and fwctl did as
>well. So maybe iommufd is weird.

Well, one CTLV chardev is always backed by one device. I think this 1:1
mapping is clear in the device model. Iommufd indeed looks weird in this
matter.


>
>> >> >What about userspace, it "knows" somehow what to do?
>> >> 
>> >> Userspace sees the family chain. It also has ability to do introspection
>> >> seeing right away which op supports with attrs in which nests.
>> >
>> >I was hoping the introspection would not be essential to use the
>> >interface..
>> 
>> Depends on the user/subsystem. Either families_dump op could be used of
>> udev uvevents that also carry family inheritance chain:
>>     CTLV_FAMILY=ctlv-attestation-tdx
>>     CTLV_COMPATIBLE_0=ctlv
>>     CTLV_COMPATIBLE_1=ctlv-attestation
>>     CTLV_COMPATIBLE_2=ctlv-attestation-tdx
>>     CTLV_COMPATIBLE_N=3
>
>I wouldn't try to link it to udev either, by the time some userspace
>uses the cdev the udev information is gone, and may be unreachable
>from inside a container or similar..

This just gives possibility for udev to work properly. Depends on
the user/usecase if it's going to be used or not. Not sure what you mean
by "udev information is gone". Udev might use these uevent info to do
aliases, similar to dev/disk/by-*


>
>So there should probably be documentation in the subsystem how
>userspace should learn the driver family via introspection.

Okay.

>
>Jason