Re: [Xen-devel] [PATCH v7 16/18] xen: automatically create XenBlockDevice-s

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf
> Of Paul Durrant
> Sent: 08 January 2019 14:11
> To: Anthony Perard <anthony.perard@xxxxxxxxxx>
> Cc: 'Kevin Wolf' <kwolf@xxxxxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; qemu-block@xxxxxxxxxx; qemu-devel@xxxxxxxxxx;
> Max Reitz <mreitz@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v7 16/18] xen: automatically create
> XenBlockDevice-s
> 
> > -----Original Message-----
> > From: Anthony PERARD [mailto:anthony.perard@xxxxxxxxxx]
> > Sent: 08 January 2019 13:28
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: 'Kevin Wolf' <kwolf@xxxxxxxxxx>; qemu-devel@xxxxxxxxxx; qemu-
> > block@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Max Reitz
> > <mreitz@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Subject: Re: [PATCH v7 16/18] xen: automatically create XenBlockDevice-s
> >
> > On Tue, Jan 08, 2019 at 01:07:49PM +0000, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Kevin Wolf [mailto:kwolf@xxxxxxxxxx]
> > > > Sent: 08 January 2019 12:53
> > > > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > > > Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>; qemu-
> devel@xxxxxxxxxx;
> > > > qemu-block@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Max Reitz
> > > > <mreitz@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > > Subject: Re: [PATCH v7 16/18] xen: automatically create
> > XenBlockDevice-s
> > > >
> > > > Am 04.01.2019 um 17:40 hat Paul Durrant geschrieben:
> > > > > > -----Original Message-----
> > > > > > From: Anthony PERARD [mailto:anthony.perard@xxxxxxxxxx]
> > > > > > Sent: 04 January 2019 16:31
> > > > > > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > > > > > Cc: qemu-devel@xxxxxxxxxx; qemu-block@xxxxxxxxxx; xen-
> > > > > > devel@xxxxxxxxxxxxxxxxxxxx; Kevin Wolf <kwolf@xxxxxxxxxx>; Max
> > Reitz
> > > > > > <mreitz@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > > > > Subject: Re: [PATCH v7 16/18] xen: automatically create
> > > > XenBlockDevice-s
> > > > > >
> > > > > > Almost done, there is one thing left which I believe is an
> issue.
> > > > > > Whenever I attach a raw file to QEMU, it print:
> > > > > >     qemu-system-i386: warning: Opening a block device as a file
> > using
> > > > the
> > > > > > 'file' driver is deprecated
> > > > >
> > > > > Oh, I'd not noticed that... but then I only use raw files
> > occasionally.
> > > >
> > > > Strictly speaking, this is not about raw (regular) files, but raw
> > block
> > > > devices. 'file' is fine for actual regular files, but the protocol
> > > > driver for block devices is 'host_device'.
> > > >
> > > > > > raw files should use the "raw" driver, so we aren't done yet.
> > > > >
> > > > > Ok. Having a strictly 2-layer stack actually makes things simpler
> > anyway
> > > > :-)
> > > >
> > > > Using 'raw' there will make the block layer auto-detect the right
> > > > protocol layer, so this works. If you want to avoid the second
> layer,
> > > > you'd have to figure out manually whether to use 'file' or
> > > > 'host_device'.
> > >
> > > Thanks for the explanation. I'll give it a spin using a device... I've
> > posted v8 but, given what you say, I'm still not sure I have it right.
> >
> > Indeed, in v8, even with the extra 'raw' layer, the warning is still
> > there. I was trying to understand why, and I only found out today that
> > we would need to use the 'host_device' driver as explain by Kevin.
> >
> >
> > BTW Paul, we can simplify the code that manage layers, by not managing
> > them.
> > Instead of (in JSON / QMP term):
> >     { 'driver': 'file', 'filename': '/file', 'node-name': 'node-file' }
> >     { 'driver': 'qcow2', 'file': 'node-file', 'node-name': 'node-qcow2'
> }
> > we can have:
> >     { 'driver': 'qcow2', 'node-name': 'node-qcow2',
> >       'file': { 'driver': 'file', 'filename': '/file' } }
> >
> 
> I kind of like the clean separation though... From what Kevin said, it
> sounds like the lowest layer should use 'raw' instead of 'file' to DTRT,
> and then we should be back to only needing the single layer in that case.
> I'll revert back to v7 and give it a try.

No, that doesn't work as we can't deal with locking correctly unless we use 
driver=file so maybe I misunderstood.

  Paul

> 
>   Paul
> 
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