Am 08.11.2018 um 15:00 hat Paul Durrant geschrieben: > > -----Original Message----- > > From: Markus Armbruster [mailto:arm...@redhat.com] > > Sent: 05 November 2018 15:58 > > To: Paul Durrant <paul.durr...@citrix.com> > > Cc: 'Kevin Wolf' <kw...@redhat.com>; Tim Smith <tim.sm...@citrix.com>; > > Stefano Stabellini <sstabell...@kernel.org>; qemu-block@nongnu.org; qemu- > > de...@nongnu.org; Max Reitz <mre...@redhat.com>; Anthony Perard > > <anthony.per...@citrix.com>; xen-de...@lists.xenproject.org > > Subject: Re: [Qemu-devel] xen_disk qdevification > > > > Paul Durrant <paul.durr...@citrix.com> writes: > > > > >> -----Original Message----- > > >> From: Kevin Wolf [mailto:kw...@redhat.com] > > >> Sent: 02 November 2018 11:04 > > >> To: Tim Smith <tim.sm...@citrix.com> > > >> Cc: xen-de...@lists.xenproject.org; qemu-de...@nongnu.org; qemu- > > >> bl...@nongnu.org; Anthony Perard <anthony.per...@citrix.com>; Paul > > Durrant > > >> <paul.durr...@citrix.com>; Stefano Stabellini <sstabell...@kernel.org>; > > >> Max Reitz <mre...@redhat.com>; arm...@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. 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. 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. Kevin
