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

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Date:   Tue, 10 Mar 2020 21:08:56 +0000
From:   John Garry <john.garry@...wei.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     <axboe@...nel.dk>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <hare@...e.de>,
        <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 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

"

 >> My implementation actually allows for per-device reserved tags (eg for
 >> virtio). But some drivers require to use internal commands prior to any
 >> device setup, so they have to use a separate reserved command queue 
just to
 >> be able to allocate tags.
 >
 > Why would virtio-scsi need per-device reserved commands?  It 
currently uses
 > a global mempool to allocate the reset commands.
 >
Oh, I'm perfectly fine with dropping the per-device reserved commands,
and use the host-wide queue in general.
It turns out most of the drivers use it that way already.
Will be doing so for the next iteration.

"

Cheers