Re: Libvirt NVME support
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.