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

[John Garry <john.garry@huawei.com>]

On 12/03/2021 16:08, Hannes Reinecke wrote:
> 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:

If I'm not mistaken, init_cmd_priv has no in-tree user today. Any plans 
to add one ... to see what it's about?

Thanks,
John

>>
>> 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
>