Re: [PATCH v9 12/22] s390: vfio-ap: sysfs interfaces to configure control domains

From: Tony Krowiak
Date: Thu Aug 23 2018 - 10:17:12 EST

On 08/23/2018 06:25 AM, Cornelia Huck wrote:
On Wed, 22 Aug 2018 15:16:19 -0400
Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:

One of the things I suggested in a private conversation with Christian
today was to provide an additional rw sysfs attribute - a boolean - that
whether all usage domains should also be control domains. The default
could be
true. This would allow one to configure guests with usage-only domains
as well
as satisfy the convention.
Would this additional attribute then control "add usage domains to the
list of control domains automatically", or "don't allow to add a usage
domain if it has not already been added as a control domain"?

It was just a proposal that wasn't really discussed at all, but this
attribute would add usage domains to the list of control domains
automatically if set to one. That would be the default behavior which
would be turned off by manually setting it to zero.

One thing I'm still unsure about is how libvirt comes into the picture
here. Will it consume the setting, or actively manipulate it?

libvirt is not in the picture. The only thing libvirt does is identify
which mediated matrix device (i.e., UUID) is to be used by the guest.

[In general, I'm not very clear about how libvirt will interact with the
whole infrastructure...]

The libvirt domain XML will only specify the UUID of the matrix device
to be used by the guest. Consequently, libvirt will insert the device
statement for the vfio-ap device into the QEMU command line:

-device vfio-ap,syfsdev={path-to-mdev}

All configuration of the mediated device, including the proposed attribute
above, is handled via sysfs attributes.