Re: [Qemu-devel] xen_disk qdevification

[Kevin Wolf <kwolf@redhat.com>]

Am 09.11.2018 um 11:27 hat Paul Durrant geschrieben:
> > -----Original Message-----
> > From: Paul Durrant
> > Sent: 08 November 2018 16:44
> > To: Paul Durrant <Paul.Durrant@citrix.com>; 'Kevin Wolf'
> > <kwolf@redhat.com>
> > Cc: Stefano Stabellini <sstabellini@kernel.org>; qemu-block@nongnu.org;
> > Tim Smith <tim.smith@citrix.com>; qemu-devel@nongnu.org; 'Markus
> > Armbruster' <armbru@redhat.com>; Anthony Perard
> > <anthony.perard@citrix.com>; xen-devel@lists.xenproject.org; Max Reitz
> > <mreitz@redhat.com>
> > Subject: RE: [Qemu-devel] xen_disk qdevification
> > 
> > > -----Original Message-----
> > > From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On
> > Behalf
> > > Of Paul Durrant
> > > Sent: 08 November 2018 15:44
> > > To: 'Kevin Wolf' <kwolf@redhat.com>
> > > Cc: Stefano Stabellini <sstabellini@kernel.org>; qemu-block@nongnu.org;
> > > Tim Smith <tim.smith@citrix.com>; qemu-devel@nongnu.org; 'Markus
> > > Armbruster' <armbru@redhat.com>; Anthony Perard
> > > <anthony.perard@citrix.com>; xen-devel@lists.xenproject.org; Max Reitz
> > > <mreitz@redhat.com>
> > > Subject: Re: [Xen-devel] [Qemu-devel] xen_disk qdevification
> > >
> > > > -----Original Message-----
> > > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > > Sent: 08 November 2018 15:21
> > > > To: Paul Durrant <Paul.Durrant@citrix.com>
> > > > Cc: 'Markus Armbruster' <armbru@redhat.com>; Anthony Perard
> > > > <anthony.perard@citrix.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>; xen-
> > > > devel@lists.xenproject.org
> > > > Subject: Re: [Qemu-devel] xen_disk qdevification
> > > >
> > > > Am 08.11.2018 um 15:00 hat Paul Durrant geschrieben:
> > > > > > -----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?
> > > >
> > > > I think the clean solution is to keep the BlockBackend open in xen-
> > disk
> > > > from the beginning, but not requesting write permissions yet.
> > > >
> > > > The BlockBackend is created in parse_drive(), when qdev parses the
> > > > -device drive=... option. At this point, no permissions are requested
> > > > yet. That is done in blkconf_apply_backend_options(), which is
> > manually
> > > > called from the devices; specifically from ide_dev_initfn() in IDE,
> > and
> > > > I assume you call the function from xen-disk as well.
> > >
> > > Yes, I call it during realize.
> > >
> > > >
> > > > xen-disk should then call this function with readonly=true, and at the
> > > > point of the handover (when the IDE device is already gone) it can
> > call
> > > > blk_set_perm() to request BLK_PERM_WRITE in addition to the
> > permissions
> > > > it already holds.
> > > >
> > >
> > > I tried that and it works fine :-)
> > 
> > Unfortunately I spoke too soon... I still had a patch in place to disable
> > locking checks :-(
> > 
> > What I'm trying to do to maintain compatibility with the existing Xen
> > toolstack (which I think is the only feasible way to make the change
> > avoiding chicken and egg problems) is to use a 'compat' function that
> > creates a drive based on the information that the Xen toolstack writes
> > into xenstore. I'm using drive_new() to do this and it is this that fails.
> > 
> > So, I have tried setting BDRV_OPT_READ_ONLY and BDRV_OPT_FORCE_SHARE. This
> > allows me to get through drive_new() but later I fail to set the write
> > permission with error "Block node is read-only".

drive_new() is really a legacy interface. It immediately creates a
BlockBackend and takes permissions for it. You don't want that here.
(And I'd like it to go away in a few releases, so better don't let new
code rely on it.)

If you can, it would be better to just call qmp_blockdev_add(). This
creates only a node (BlockDriverState) without a BlockBackend. You'll
get your BlockBackend from the qdev drive property.

> > > > The other option I see would be that you simply create both devices
> > with
> > > > share-rw=on (which results in conf->share_rw == true and therefore
> > > > shared BLK_PERM_WRITE in blkconf_apply_backend_options()), but that
> > > > feels like a hack because you don't actually want to have two writers
> > at
> > > > the same time.
> > > >
> > >
> > > Yes, that does indeed seem like more of a hack. The first option works
> > so
> > > I'll go with that.
> > >
> > 
> > I'll now see what I can do with this idea.
> 
> I think I have a reasonably neat solution, as it is restricted to my
> compat code and can thus go away when the Xen toolstack is re-educated
> to use QMP to instantiate PV backends (once they are all qdevified). I
> simply add "file.locking=off" to the options I pass to drive_new().

I wouldn't agree on "neat", but if you think so...

Kevin

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