Re: [Qemu-devel] [Qemu-block] [RFC] nvme: how to support multiple namespaces

[Klaus Birkelund <klaus@birkelund.eu>]

On Mon, Jun 24, 2019 at 12:18:45PM +0200, Kevin Wolf wrote:
> Am 24.06.2019 um 10:01 hat Klaus Birkelund geschrieben:
> > On Thu, Jun 20, 2019 at 05:37:24PM +0200, Laszlo Ersek wrote:
> > > On 06/17/19 10:12, Klaus Birkelund wrote:
> > > > Hi all,
> > > > 
> > > > I'm thinking about how to support multiple namespaces in the NVMe
> > > > device. My first idea was to add a "namespaces" property array to the
> > > > device that references blockdevs, but as Laszlo writes below, this might
> > > > not be the best idea. It also makes it troublesome to add per-namespace
> > > > parameters (which is something I will be required to do for other
> > > > reasons). Some of you might remember my first attempt at this that
> > > > included adding a new block driver (derived from raw) that could be
> > > > given certain parameters that would then be stored in the image. But I
> > > > understand that this is a no-go, and I can see why.
> > > > 
> > > > I guess the optimal way would be such that the parameters was something
> > > > like:
> > > > 
> > > >    -blockdev raw,node-name=blk_ns1,file.driver=file,file.filename=blk_ns1.img
> > > >    -blockdev raw,node-name=blk_ns2,file.driver=file,file.filename=blk_ns2.img
> > > >    -device nvme-ns,drive=blk_ns1,ns-specific-options (nsfeat,mc,dlfeat)...
> > > >    -device nvme-ns,drive=blk_ns2,...
> > > >    -device nvme,...
> > > > 
> > > > My question is how to state the parent/child relationship between the
> > > > nvme and nvme-ns devices. I've been looking at how ide and virtio does
> > > > this, and maybe a "bus" is the right way to go?
> > > 
> > > I've added Markus to the address list, because of this question. No
> > > other (new) comments from me on the thread starter at this time, just
> > > keeping the full context.
> > > 
> > 
> > Hi all,
> > 
> > I've succesfully implemented this by introducing a new 'nvme-ns' device
> > model. The nvme device creates a bus named from the device id ('id'
> > parameter) and the nvme-ns devices are then registered on this.
> > 
> > This results in an nvme device being creates like this (two namespaces
> > example):
> > 
> >   -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
> > 
> > How does that look as a way forward?
> 
> This looks very similar to what other devices do (one bus controller
> that has multiple devices on its but), so I like it.
> 
> The thing that is special here is that -device nvme is already a block
> device by itself that can take a drive property. So how does this play
> together? Can I choose to either specify a drive directly for the nvme
> device or nvme-ns devices, but when I do both, I will get an error? What
> happens if I don't specify a drive for nvme, but also don't add nvme-ns
> devices?
> 

Hi Kevin,

Yes, the nvme device is already a block device. My current patch removes
that property from the nvme device. I guess this breaks backward
compatibiltiy. We could accept a drive for the nvme device only if no
nvme-ns devices are configured and connected on the bus.

I'm not entirely sure on the spec, but my gut tells me that an nvme
device without any namespaces is technically a valid device, although it
is a bit useless.

I will post my patch (as part of a larger series) and we can discuss it
there.

Thanks for the feedback!

Klaus