Re: xen_disk qdevification (was: [PATCH 0/3] Performance improvements for xen_disk v2)

From: Paul Durrant <Paul.Durrant@citrix.com>
To: 'Kevin Wolf' <kwolf@redhat.com>, Tim Smith <tim.smith@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>,
	"armbru@redhat.com" <armbru@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	Max Reitz <mreitz@redhat.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: xen_disk qdevification (was: [PATCH 0/3] Performance improvements for xen_disk v2)
Date: Fri, 2 Nov 2018 11:13:48 +0000	[thread overview]
Message-ID: <152b8d37c7d54de687f4ced536ba6a10__20287.6992793337$1541157170$gmane$org@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <20181102110412.GD7521@dhcp-200-186.str.redhat.com>

> -----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.

  Paul

> Kevin

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