[dm-devel] [RFC PATCH 3/3] nvme: add the "debug" host driver

[Javier González]

On 04.02.2022 07:58, Chaitanya Kulkarni wrote:
>On 2/3/22 22:28, Damien Le Moal wrote:
>> On 2/4/22 12:12, Chaitanya Kulkarni wrote:
>>>
>>>>>> One can instantiate scsi devices with qemu by using fake scsi devices,
>>>>>> but one can also just use scsi_debug to do the same. I see both efforts
>>>>>> as desirable, so long as someone mantains this.
>>>>>>
>>>
>>> Why do you think both efforts are desirable ?
>>
>> When testing code using the functionality, it is far easier to get said
>> functionality doing a simple "modprobe" rather than having to setup a
>> VM. C.f. running blktests or fstests.
>>
>
>agree on simplicity but then why do we have QEMU implementations for
>the NVMe features (e.g. ZNS, NVMe Simple Copy) ? we can just build
>memoery backed NVMeOF test target for NVMe controller features.
>
>Also, recognizing the simplicity I proposed initially NVMe ZNS
>fabrics based emulation over QEMU (I think I still have initial state
>machine implementation code for ZNS somewhere), those were "nacked" for
>the right reason, since we've decided go with QEMU and use that as a
>primary platform for testing, so I failed to understand what has
>changed.. since given that QEMU already supports NVMe simple copy ...

I was not part of this conversation, but as I see it each approach give
a benefit. QEMU is fantastic for compliance testing and I am not sure
you get the same level of command analysis anywhere else; at least not
without writing dedicated code for this in a target.

This said, when we want to test for race conditions, QEMU is very slow.
For a software-only solution, we have experimented with something
similar to the nvme-debug code tha Mikulas is proposing. Adam pointed to
the nvme-loop target as an alternative and this seems to work pretty
nicely. I do not believe there should be many changes to support copy
offload using this.

So in my view having both is not replication and it gives more
flexibility for validation, which I believe it is always good.

>
>> So personally, I also think it would be great to have a kernel-based
>> emulation of copy offload. And that should be very easy to implement
>> with the fabric code. Then loopback onto a nullblk device and you get a
>> quick and easy to setup copy-offload device that can even be of the ZNS
>> variant if you want since nullblk supports zones.
>>
>
>One can do that with creating null_blk based NVMeOF target namespace,
>no need to emulate simple copy memory backed code in the fabrics
>with nvme-loop.. it is as simple as inserting module and configuring
>ns with nvmetcli once we have finalized the solution for copy offload.
>If you remember, I already have patches for that...
>
>>>
>>> NVMe ZNS QEMU implementation proved to be perfect and works just
>>> fine for testing, copy offload is not an exception.
>>>
>>>>>> For instance, blktests uses scsi_debug for simplicity.
>>>>>>
>>>>>> In the end you decide what you want to use.
>>>>>
>>>>> Can we use the nvme-loop target instead?
>>>>
>>>> I am advocating for this approach as well. It presentas a virtual nvme
>>>> controller already.
>>>>
>>>
>>> It does that assuming underlying block device such as null_blk or
>>> QEMU implementation supports required features not to bloat the the
>>> NVMeOF target.
>>>
>>> -ck
>>>
>
>-ck
>