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

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Date:   Tue, 7 Apr 2020 16:45:04 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     John Garry <john.garry@...wei.com>,
        Christoph Hellwig <hch@...radead.org>
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 4/7/20 4:35 PM, John Garry wrote:
> On 07/04/2020 15:00, Hannes Reinecke wrote:
>> On 4/7/20 1:54 PM, John Garry wrote:
>>> On 06/04/2020 10:05, Hannes Reinecke wrote:
[ .. ]
>>>> 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.
>>>
>>> wouldn't that limit 1x scsi device per host, not that I know if any 
>>> more would ever be required? But it does still seem better to use the 
>>> request queue in the scsi device.
>>>
>> My concern is this:
>>
>> struct scsi_device *scsi_get_host_dev(struct Scsi_Host *shost)
>> {
>>      [ .. ]
>>      starget = scsi_alloc_target(&shost->shost_gendev, 0, 
>> shost->this_id);
>>      [ .. ]
>>
>> and we have typically:
>>
>> drivers/scsi/hisi_sas/hisi_sas_v3_hw.c: .this_id                = -1,
>>
>> It's _very_ uncommon to have a negative number as the SCSI target 
>> device; in fact, it _is_ an unsigned int already.
>>
> 
> FWIW, the only other driver (gdth) which I see uses this API has this_id 
> = -1 in the scsi host template.
> 
>> But alright, I'll give it a go; let's see what I'll end up with.
> 
> note: If we want a fixed scsi_device per host, calling 
> scsi_mq_setup_tags() -> scsi_get_host_dev() will fail as shost state is 
> not running. Maybe we need to juggle some things there to provide a 
> generic solution.
> 
It might even get worse, as during device setup things like 
'slave_alloc' etc is getting called, which has a fair chance of getting 
confused for non-existing devices.
Cf qla2xxx:qla2xx_slave_alloc() is calling starget_to_rport(), which 
will get us a nice oops when accessing a target which is _not_ the child 
of a fc remote port.
And this is why I'm not utterly keen on this approach; auditing all 
these callbacks is _not_ fun.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@...e.de                               +49 911 74053 688
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