linux-kernel - Re: [PATCH v2 3/5] vfio-pci/zdev: define the vfio

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Date:   Mon, 5 Oct 2020 18:01:07 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        schnelle@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] vfio-pci/zdev: define the vfio_zdev header

On Mon, 5 Oct 2020 09:52:25 -0400
Matthew Rosato <mjrosato@...ux.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.