linux-kernel - Re: [PATCH v15 7/9] nvmet-passthru: Add passthru code to process commands

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Message-ID: <6e8cc582-d706-8e99-7191-82b08aa557e2@grimberg.me>
Date:   Wed, 22 Jul 2020 15:31:34 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>
Cc:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Stephen Bates <sbates@...thlin.com>, Jens Axboe <axboe@...com>,
        Keith Busch <kbusch@...nel.org>,
        Max Gurtovoy <maxg@...lanox.com>
Subject: Re: [PATCH v15 7/9] nvmet-passthru: Add passthru code to process
 commands


>>> Thanks for the review Christoph. I think I should be able to make all
>>> the requested changes in the next week or two.
>>>
>>> On 2020-07-20 1:35 p.m., Sagi Grimberg wrote:
>>>>
>>>>> I'm still not so happy about having to look up the namespace and still
>>>>> wonder if we should generalize the connect_q to a passthrough_q.  But
>>>>> I guess we can do that later and then reduce some of the exports here..
>>>>
>>>> That is a neat idea! should be easy to do (and we can then lose the host
>>>> xarray stuff). I don't mind having it on a later patch, but it should be
>>>> easy enough to do even before...
>>>>
>>>
>>> I sort of follow this. I can try to work something up but it will
>>> probably take me a few iterations to get it to where you want it. So,
>>> roughly, we'd create a passthrough_q in core with the controller's IO
>>> tagset and then cleanup the fabrics hosts to use that instead of each
>>> independently creating their connect_q?
>>>
>>> Though, I don't understand how this relates to the host xarray stuff
>>> that Sagi mentioned...
>>
>> passthru commands are in essence REQ_OP_DRV_IN/REQ_OP_DRV_OUT, which
>> means that the driver shouldn't need the ns at all. So if you have a
>> dedicated request queue (mapped to the I/O tagset), you don't need the
>> ns->queue and we can lose the ns lookup altogether.
>>
>> The only part is to check the effects, but that can probably be handled
>> when we setup the passthru controller or something...
> 
> Yes, I implemented the passthru_q (which was quite simple).

Nice..

> But I'm not
> sure how we are supposed to call nvme_command_effects() correctly
> without the ns. You can't possibly do that during setup for every
> possible opcode on every namespace. And even if we do, we'll still need
> the same nvme_find_get_ns() and nvme_put_ns() exports and probably
> another xarray to lookup the information.
> 
> Also, we pass the namespace's disk to in order to get proper block
> accounting for the underlying disk. (Which is pretty important for
> debugging). So we need to lookup the namespace for this too.
> 
> Unless there are some other ideas to solve these issues, I don't think
> this change will gain us anything.

Let's defer it to a followup set than.