Re: [PATCHv7 00/31] scsi: enable reserved commands for LLDDs

From: Hannes Reinecke <hare@suse.de>
To: michael.christie@oracle.com,
	"Martin K. Petersen" <martin.petersen@oracle.com>
Cc: James Bottomley <james.bottomley@hansenpartnership.com>,
	Christoph Hellwig <hch@lst.de>,
	John Garry <john.garry@huawei.com>,
	linux-scsi@vger.kernel.org
Subject: Re: [PATCHv7 00/31] scsi: enable reserved commands for LLDDs
Date: Fri, 12 Mar 2021 17:08:57 +0100	[thread overview]
Message-ID: <02a97468-4dc2-f840-06a9-1a3a7c5a6852@suse.de> (raw)
In-Reply-To: <ef8fe68b-dcf0-6092-b51f-1ef79af61cc2@oracle.com>

On 3/12/21 12:53 AM, michael.christie@oracle.com wrote:
> On 2/22/21 7:23 AM, Hannes Reinecke wrote:
>> Hi all,
>>
>> quite some drivers use internal commands for various purposes, most
>> commonly sending TMFs or querying the HBA status.
>> While these commands use the same submission mechanism than normal
>> I/O commands, they will not be counted as outstanding commands,
>> requiring those drivers to implement their own mechanism to figure
>> out outstanding commands.
>> The block layer already has the concept of 'reserved' tags for
>> precisely this purpose, namely non-I/O tags which live off a separate
>> tag pool. That guarantees that these commands can always be sent,
>> and won't be influenced by tag starvation from the I/O tag pool.
>> This patchset enables the use of reserved tags for the SCSI midlayer
>> by allocating a virtual LUN for the HBA itself which just serves
>> as a resource to allocate valid tags from.
>> This removes quite some hacks which were required for some
>> drivers (eg. fnic or snic), and allows the use of tagset
>> iterators within the drivers.
>>
> 
> Hey Hannes,
> 
> I was trying to port some iscsi patches to this set. One question I had
> is how to handle if my driver implements init_cmd_priv, and wants to use
> the reserved cmds for a non scsi IO. My case I want to use them for cmds
> like a iscsi nop/ping, device/target reset or login request.
> 
> There is no bit to way to tell if at init_cmd_priv time the cmd will be
> for a reserved or non reserved cmd right? If not, I was wondering should
> I do:
> 
> 1. in libiscsi, allocate an array of size $reserved_cmds with non_scsi_cmds
> structs. When I need to do a non scsi cmd do blk_mq_get_tag on the host's
> tags to get a reserved tag then use that to lookup a struct in my array?
> 
> 2. in libiscsi when I need to do a non scsi cmd do a scsi_get_internal_cmd.
> At this time allocate the non_scsi_cmd struct parts.
> 
You sure you will need to allocate additional stuff?
The request already comes with the request, scsi, and driver private
bits (ie the additional space from .cmd_size) allocated.

And yes, you can tell in init_cmd_priv() if the command is coming from
the private pool; I had a helper 'req_is_reserved' once, I thought it's
still there ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
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