linux-kernel - Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands

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Message-ID: <b5a63725-722b-8ccd-3867-6db192a248a4@suse.de>
Date:   Mon, 6 Apr 2020 11:05:34 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Christoph Hellwig <hch@...radead.org>,
        John Garry <john.garry@...wei.com>
Cc:     axboe@...nel.dk, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        ming.lei@...hat.com, bvanassche@....org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        esc.storagedev@...rosemi.com, chenxiang66@...ilicon.com,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved
 commands

On 3/11/20 7:22 AM, Christoph Hellwig wrote:
> On Tue, Mar 10, 2020 at 09:08:56PM +0000, John Garry wrote:
>> On 10/03/2020 18:32, Christoph Hellwig wrote:
>>> On Wed, Mar 11, 2020 at 12:25:28AM +0800, John Garry wrote:
>>>> From: Hannes Reinecke <hare@...e.com>
>>>>
>>>> Allocate a separate 'reserved_cmd_q' for sending reserved commands.
>>>
>>> Why?  Reserved command specifically are not in any way tied to queues.
>>> .
>>>
>>
>> So the v1 series used a combination of the sdev queue and the per-host
>> reserved_cmd_q. Back then you questioned using the sdev queue for virtio
>> scsi, and the unconfirmed conclusion was to use a common per-host q. This is
>> the best link I can find now:
>>
>> https://www.mail-archive.com/linux-scsi@vger.kernel.org/msg83177.html
> 
> That was just a question on why virtio uses the per-device tags, which
> didn't look like it made any sense.  What I'm worried about here is
> mixing up the concept of reserved tags in the tagset, and queues to use
> them.  Note that we already have the scsi_get_host_dev to allocate
> a scsi_device and thus a request_queue for the host itself.  That seems
> like the better interface to use a tag for a host wide command vs
> introducing a parallel path.
> 
Thinking about it some more, I don't think that scsi_get_host_dev() is
the best way of handling it.
Problem is that it'll create a new scsi_device with <hostno:this_id:0>,
which will then show up via eg 'lsscsi'.
This would be okay if 'this_id' would have been defined by the driver;
sadly, most drivers which are affected here do set 'this_id' to -1.
So we wouldn't have a nice target ID to allocate the device from, let
alone the problem that we would have to emulate a complete scsi device
with all required minimal command support etc.
And I'm not quite sure how well that would play with the exising SCSI
host template; the device we'll be allocating would have basically
nothing in common with the 'normal' SCSI devices.

What we could do, though, is to try it the other way round:
Lift the request queue from scsi_get_host_dev() into the scsi host
itself, so that scsi_get_host_dev() can use that queue, but we also
would be able to use it without a SCSI device attached.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@...e.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer