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

[Markus Armbruster <armbru@redhat.com>]

Laszlo Ersek <lersek@redhat.com> writes:

> 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.

Devices can be wired together without a bus intermediary.  You
definitely want a bus when the physical connection you model has one.
If not, a bus may be useful anyway, say because it provides a convenient
way to encapsulate the connection model, or to support -device bus=...

> +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.

Management applications should move to -blockdev.  -drive has too much
bad magic sticking to it.

We're not urging humans to switch, at least not yet.  We may want to
provide convenience features on top of plain -blockdev before we do.

As far as I know, we don't yet eschew -drive in documentation or commit
messages.  Perhaps we should consider such a policy for documentation.

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