Re: [Xen-devel] [Qemu-devel] xen_disk qdevification

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
> Of Paul Durrant
> Sent: 08 November 2018 15:44
> To: 'Kevin Wolf' <kwolf@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; qemu-block@xxxxxxxxxx;
> Tim Smith <tim.smith@xxxxxxxxxx>; qemu-devel@xxxxxxxxxx; 'Markus
> Armbruster' <armbru@xxxxxxxxxx>; Anthony Perard
> <anthony.perard@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Max Reitz
> <mreitz@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [Qemu-devel] xen_disk qdevification
> 
> > -----Original Message-----
> > From: Kevin Wolf [mailto:kwolf@xxxxxxxxxx]
> > Sent: 08 November 2018 15:21
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: 'Markus Armbruster' <armbru@xxxxxxxxxx>; Anthony Perard
> > <anthony.perard@xxxxxxxxxx>; Tim Smith <tim.smith@xxxxxxxxxx>; Stefano
> > Stabellini <sstabellini@xxxxxxxxxx>; qemu-block@xxxxxxxxxx; qemu-
> > devel@xxxxxxxxxx; Max Reitz <mreitz@xxxxxxxxxx>; xen-
> > devel@xxxxxxxxxxxxxxxxxxxx
> > 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@xxxxxxxxxx]
> > > > Sent: 05 November 2018 15:58
> > > > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > > > Cc: 'Kevin Wolf' <kwolf@xxxxxxxxxx>; Tim Smith
> <tim.smith@xxxxxxxxxx>;
> > > > Stefano Stabellini <sstabellini@xxxxxxxxxx>; qemu-block@xxxxxxxxxx;
> > qemu-
> > > > devel@xxxxxxxxxx; Max Reitz <mreitz@xxxxxxxxxx>; Anthony Perard
> > > > <anthony.perard@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > > Subject: Re: [Qemu-devel] xen_disk qdevification
> > > >
> > > > Paul Durrant <Paul.Durrant@xxxxxxxxxx> writes:
> > > >
> > > > >> -----Original Message-----
> > > > >> From: Kevin Wolf [mailto:kwolf@xxxxxxxxxx]
> > > > >> Sent: 02 November 2018 11:04
> > > > >> To: Tim Smith <tim.smith@xxxxxxxxxx>
> > > > >> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-devel@xxxxxxxxxx; qemu-
> > > > >> block@xxxxxxxxxx; Anthony Perard <anthony.perard@xxxxxxxxxx>;
> Paul
> > > > Durrant
> > > > >> <Paul.Durrant@xxxxxxxxxx>; Stefano Stabellini
> > <sstabellini@xxxxxxxxxx>;
> > > > >> Max Reitz <mreitz@xxxxxxxxxx>; armbru@xxxxxxxxxx
> > > > >> 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".

> 
> >
> > 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.

 Paul

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