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

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Date:   Tue, 7 Apr 2020 16:19:35 +0100
From:   John Garry <john.garry@...wei.com>
To:     Hannes Reinecke <hare@...e.de>,
        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

>>>
>>
>> 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.

Yes, something similar happens for libsas [hence my hack], where 
sas_alloc_target()->sas_find_dev_by_rphy() fails as it cannot handle 
rphy for scsi host as parent properly.

> And this is why I'm not utterly keen on this approach; auditing all 
> these callbacks is _not_ fun.
> 

Understood. And if you can't test them, then a change like this is too 
risky for those drivers.

Cheers,
John