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SubjectRe: [PATCH v2 3/5] vfio-pci/zdev: define the vfio_zdev header
On Mon, 5 Oct 2020 09:52:25 -0400
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> On 10/2/20 5:44 PM, Alex Williamson wrote:

> > Can you discuss why a region with embedded capability chain is a better
> > solution than extending the VFIO_DEVICE_GET_INFO ioctl to support a
> > capability chain and providing this info there? This all appears to be
> > read-only info, so what's the benefit of duplicating yet another
>
> It is indeed read-only info, and the device region was defined as such.
>
> I would not necessarily be opposed to extending VFIO_DEVICE_GET_INFO
> with these defined as capabilities; I'd say a primary motivating factor
> to putting these in their own region was to avoid stuffing a bunch of
> s390-specific capabilities into a general-purpose ioctl response.

Can't you make the zdev code register the capabilities? That would put
them nicely into their own configurable part.

>
> But if you're OK with that notion, I can give that a crack in v3.
>
> > capability chain in a region? It would also be possible to define four
> > separate device specific regions, one for each of these capabilities
> > rather than creating this chain. It just seems like a strange approach
>
> I'm not sure if creating separate regions would be the right approach
> though; these are just the first 4. There will definitely be additional
> capabilities in support of new zPCI features moving forward, I'm not
> sure how many regions we really want to end up with. Some might be as
> small as a single field, which seems more in-line with capabilities vs
> an entire region.

If we are expecting more of these in the future, going with GET_INFO
capabilities when adding new ones seems like the best approach.

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