Libvirt NVME support

[Peter Krempa]

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.

> 	
> > > >
> > > > The 'drive' address here maps the disk to the controller. This example
> 
> IIUC we need a way to associate storage (this XML snippet) with the controller
> you defined earlier (<controller type='nvme' index='1' model='nvme'>). So
> shouldn't we only require associating this piece of storage with the controller
> based on the index?

No. The common approach is to do it via what's specified as <address>

> 
> > > > uses unit= as the way to specify the namespace ID. Both 'bus' and 'target'
> > > > must be 0.
> 
> I think 'namespace' or 'ns' would be more suitable instead of 'unit'.
> What are 'bus' and 'target' here? And why do they have to be 0?
> Do we really need dev='nvme0' in <target ...>? Specifying the controller index
> should be enough, no?

You certainly can add <address type='nvme' controller='1' ns='2'/>

> Wouldn't this contain the minimum amount of information to unambiguously map
> this piece of storage to the controller?
> 
>     <disk type='file' device='disk'>
>       <source dev='/Host/QEMUGuest2.qcow2'/>
>       <target dev='sdb' bus='nvme'/>
>       <address controller='1' ns='1'/>
>     </disk>

That certainly is correct if you include the "type='nvme'" attribute.