[libvirt PATCH v3 00/21] Add support for persistent mediated devices

[Daniel P. Berrangé]

On Mon, Feb 01, 2021 at 10:45:43AM +0100, Erik Skultety wrote:
> On Mon, Feb 01, 2021 at 09:42:32AM +0000, Daniel P. Berrangé wrote:
> > On Fri, Jan 29, 2021 at 05:34:36PM -0600, Jonathon Jongsma wrote:
> > > On Thu, 7 Jan 2021 17:43:54 +0100
> > > Erik Skultety <eskultet at redhat.com> wrote:
> > > 
> > > > > Tested with v6.10.0-283-g1948d4e61e.
> > > > > 
> > > > > 1.Can define/start/destroy mdev device successfully;
> > > > > 
> > > > > 2.'virsh nodedev-list' has no '--active' option, which is
> > > > > inconsistent with the description in the patch：
> > > > > # virsh nodedev-list --active
> > > > > error: command 'nodedev-list' doesn't support option --active
> > > > > 
> > > > > 3.virsh client hang when trying to destroy a mdev device which is
> > > > > using by a vm, and after that all 'virsh nodev*' cmds will hang. If
> > > > > restarting llibvirtd after that, libvirtd will hang.  
> > > > 
> > > > It hangs because underneath a write to the 'remove' sysfs attribute
> > > > is now blocking for some reason and since we're relying on mdevctl to
> > > > do it for us, hence "it hangs". I'm not trying to make an excuse,
> > > > it's plain wrong. I'd love to rely on such a basic functionality, but
> > > > it looks like we'll have to go with a extremely ugly workaround and
> > > > try to get the list of active domains from the nodedev driver and see
> > > > whether any of them has the device assigned before we try to destroy
> > > > the mdev via the nodedev driver.
> > > 
> > > So, I've been trying to figure out a way to do this, but as far as
> > > I know, there's no way to get a list of active domains from within the
> > > nodedev driver, and I can't think of any better ways to handle it. Any
> > > ideas?
> > 
> > Correct, the nodedev driver isn't permitted to talk to any of the virt
> > drivers.
> 
> Oh, not even via secondary connection? What makes nodedev so special, since we
> can open a secondary connection from e.g. the storage driver?

It is technically possible, but it should never be done, because it
introduces a bi-directional dependancy between the daemons which
introduces the danger of deadlocking them. None of the secondary
drivers should connect to the hypervisor drivers.

> > Is there anything in sysfs which reports whether the device is in use ?
> 
> Nothing that I know of, the way it used to work was that you tried to write to
> sysfs and kernel returned a write error with "device in use" or something like
> that, but that has changed since :(.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|