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

[Laszlo Ersek]

On 06/24/19 12:18, 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.

+1

Also, I believe it's more modern nowadays to express the same example
with "blockdev" syntax, rather than "drive". (Not that I could suggest
the exact spelling for that :)) I don't expect the modern syntax to
behave differently, I just guess it's better to stick with the new in
examples / commit messages etc.

> 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?

Great questions!

Thanks!
Laszlo