Libvirt NVME support

[Thanos Makatos]

> On Mon, Nov 23, 2020 at 13:07:51 +0000, Thanos Makatos wrote:
> >
> > > On Mon, Nov 23, 2020 at 09:47:23 +0000, Thanos Makatos wrote:
> > > > > On Thu, Nov 19, 2020 at 10:17:56 +0000, Thanos Makatos wrote:
> > > > > > > As a starting point a trivial way to model this in the XML will be:
> > > > > > >
> > > > > > >     <controller type='nvme' index='1' model='nvme'>
> > > > > > >
> > > > > > > And then add the storage into it as:
> > > > > > >
> > > > > > >     <disk type='file' device='disk'>
> > > > > > >       <source dev='/Host/QEMUGuest1.qcow2'/>
> > > > > > >       <target dev='sda' bus='nvme'/>
> > > > > >
> > > > > > 'target dev' is how the device appears in the guest, right? It should
> be
> > > > > > something like 'nvme0n1'. I'm  not sure though this is something
> that
> > > we
> > > > > can
> > > > > > put here anyway, I think the guest driver can number controllers
> > > arbitrarily.
> > > > >
> > > > > Well, it was supposed to be like that but really is not. Even with other
> > > > > buses the kernel can name the device arbitrarily, so it doesn't really
> > > > > matter.
> > > > >
> > > > > > Maybe we should specify something like BDF? Or is this something
> > > QEMU
> > > > > will
> > > > > > have to figure out how to do?
> > > > > >
> > > > > > >       <address type='drive' controller='1' bus='0' target='0' unit='0'/>
> > > > > > >     </disk>
> > > > > > >
> > > > > > >     <disk type='file' device='disk'>
> > > > > > >       <source dev='/Host/QEMUGuest2.qcow2'/>
> > > > > > >       <target dev='sdb' bus='nvme'/>
> > > > > > >       <address type='drive' controller='1' bus='0' target='0' unit='1'/>
> > > > > > >     </disk>
> > > >
> > > > Revistiting your initial suggestion, it should be something like this
> > > > (s/sdb/nvme0):
> > > >
> > > >     <disk type='file' device='disk'>
> > > >       <source dev='/Host/QEMUGuest2.qcow2'/>
> > > >       <target dev='nvme0' bus='nvme'/>
> > > >       <address type='drive' controller='1' bus='0' target='0' unit='1'/>
> > > >     </disk>
> > >
> > > Note that the parser for 'dev' is a bit quirky, old, and used in many
> > > places besides the qemu driver. It's also used with numbers in non-qemu
> > > cases. Extending that to parse numbers for nvme but not for sda might
> > > become ugly very quickly. Sticking with a letter at the end ('nvmea'
> > > might be a more straightforward approach.
> >
> > Then I think we should just stick with 'nvme'.
> 
> You still need a way to "index" it somehow. The target must be unique
> for each disk.

I think I've misunderstood something, I thought controller='1' in <address ...>
refers to index='1' in <controller ...>. So <address ...> should be:

    <address type='nvme' index='1' controller='1' ns='2'/>

What's controller='1' then?