Re: [Qemu-devel] xen_disk qdevification

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Markus Armbruster [mailto:armbru@redhat.com]
> Sent: 05 November 2018 15:58
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: 'Kevin Wolf' <kwolf@redhat.com>; Tim Smith <tim.smith@citrix.com>;
> Stefano Stabellini <sstabellini@kernel.org>; qemu-block@nongnu.org; qemu-
> devel@nongnu.org; Max Reitz <mreitz@redhat.com>; Anthony Perard
> <anthony.perard@citrix.com>; xen-devel@lists.xenproject.org
> Subject: Re: [Qemu-devel] xen_disk qdevification
> 
> Paul Durrant <Paul.Durrant@citrix.com> writes:
> 
> >> -----Original Message-----
> >> From: Kevin Wolf [mailto:kwolf@redhat.com]
> >> Sent: 02 November 2018 11:04
> >> To: Tim Smith <tim.smith@citrix.com>
> >> Cc: xen-devel@lists.xenproject.org; qemu-devel@nongnu.org; qemu-
> >> block@nongnu.org; Anthony Perard <anthony.perard@citrix.com>; Paul
> Durrant
> >> <Paul.Durrant@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>;
> >> Max Reitz <mreitz@redhat.com>; armbru@redhat.com
> >> Subject: xen_disk qdevification (was: [PATCH 0/3] Performance
> improvements
> >> for xen_disk v2)
> >>
> >> Am 02.11.2018 um 11:00 hat Tim Smith geschrieben:
> >> > A series of performance improvements for disks using the Xen PV ring.
> >> >
> >> > These have had fairly extensive testing.
> >> >
> >> > The batching and latency improvements together boost the throughput
> >> > of small reads and writes by two to six percent (measured using fio
> >> > in the guest)
> >> >
> >> > Avoiding repeated calls to posix_memalign() reduced the dirty heap
> >> > from 25MB to 5MB in the case of a single datapath process while also
> >> > improving performance.
> >> >
> >> > v2 removes some checkpatch complaints and fixes the CCs
> >>
> >> Completely unrelated, but since you're the first person touching
> >> xen_disk in a while, you're my victim:
> >>
> >> At KVM Forum we discussed sending a patch to deprecate xen_disk because
> >> after all those years, it still hasn't been converted to qdev. Markus
> is
> >> currently fixing some other not yet qdevified block device, but after
> >> that xen_disk will be the only one left.
> >>
> >> A while ago, a downstream patch review found out that there are some
> QMP
> >> commands that would immediately crash if a xen_disk device were present
> >> because of the lacking qdevification. This is not the code quality
> >> standard I envision for QEMU. It's time for non-qdev devices to go.
> >>
> >> So if you guys are still interested in the device, could someone please
> >> finally look into converting it?
> >>
> >
> > I have a patch series to do exactly this. It's somewhat involved as I
> > need to convert the whole PV backend infrastructure. I will try to
> > rebase and clean up my series a.s.a.p.
> 
> Awesome!  Please coordinate with Anthony Prerard to avoid duplicating
> work if you haven't done so already.

I've come across a bit of a problem that I'm not sure how best to deal with and so am looking for some advice.

I now have a qdevified PV disk backend but I can't bring it up because it fails to acquire a write lock on the qcow2 it is pointing at. This is because there is also an emulated IDE drive using the same qcow2. This does not appear to be a problem for the non-qdev xen-disk, presumably because it is not opening the qcow2 until the emulated device is unplugged and I don't really want to introduce similar hackery in my new backend (i.e. I want it to attach to its drive, and hence open the qcow2, during realize).

So, I'm not sure what to do... It is not a problem that both a PV backend and an emulated device are using the same qcow2 because they will never actually operate simultaneously so is there any way I can bypass the qcow2 lock check when I create the drive for my PV backend? (BTW I tried re-using the drive created for the emulated device, but that doesn't work because there is a check if a drive is already attached to something).

Any ideas?

  Paul