Re: [Qemu-devel] [Qemu-block] [PATCH 16/16] nvme: support multiple namespaces

[Daniel P . Berrangé]

On Fri, Jul 05, 2019 at 06:20:14PM +0200, Klaus Birkelund wrote:
> On Fri, Jul 05, 2019 at 02:49:29PM +0100, Daniel P. Berrangé wrote:
> > On Fri, Jul 05, 2019 at 03:36:17PM +0200, Klaus Birkelund wrote:
> > > On Fri, Jul 05, 2019 at 09:23:33AM +0200, Klaus Birkelund Jensen wrote:
> > > > This adds support for multiple namespaces by introducing a new 'nvme-ns'
> > > > device model. The nvme device creates a bus named from the device name
> > > > ('id'). The nvme-ns devices then connect to this and registers
> > > > themselves with the nvme device.
> > > > 
> > > > This changes how an nvme device is created. Example with two namespaces:
> > > > 
> > > >   -drive file=nvme0n1.img,if=none,id=disk1
> > > >   -drive file=nvme0n2.img,if=none,id=disk2
> > > >   -device nvme,serial=deadbeef,id=nvme0
> > > >   -device nvme-ns,drive=disk1,bus=nvme0,nsid=1
> > > >   -device nvme-ns,drive=disk2,bus=nvme0,nsid=2
> > > > 
> > > > A maximum of 256 namespaces can be configured.
> > > > 
> > >  
> > > Well that was embarrasing.
> > > 
> > > This patch breaks nvme-test.c. Which I obviously did not run.
> > > 
> > > In my defense, the test doesn't do much currently, but I'll of course
> > > fix the test for v2.
> > 
> > That highlights a more serious problem.  This series changes the syntx
> > for configuring the nvme device in a way that is not backwards compatible.
> > So anyone who is using QEMU with NVME will be broken when they upgrade
> > to the next QEMU release.
> > 
> > I understand why you wanted to restructure things to have a separate
> > nvme-ns device, but there needs to be some backcompat support in there
> > for the existing syntax to avoid breaking current users IMHO.
> > 
>  
> Hi Daniel,
> 
> I raised this issue previously. I suggested that we keep the drive
> property for the nvme device and only accept either that or an nvme-ns
> device to be configured (but not both).
> 
> That would keep backward compatibilty, but enforce the use of nvme-ns
> for any setup that requires multiple namespaces.
> 
> Would that work?

Yes, that would be viable, as an existing CLI arg usage would continue
to be supported as before.

We could also list the back compat syntax as a deprecation in the docs
(qemu-deprecated.texi) so that in a few releases in the future, we can
drop the old syntax and then use nvme-ns exclusively thereafter.

Regards,
Daniel
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