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Subject: Re: [PATCH 1/2] scsi_host: add support for request batching
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, jejb@linux.ibm.com,
        martin.petersen@oracle.com, linux-scsi@vger.kernel.org
References: <20190530112811.3066-1-pbonzini@redhat.com>
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From:   Hannes Reinecke <hare@suse.de>
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Message-ID: <141b9bec-77e5-4b99-a494-642bc88bb1a2@suse.de>
Date:   Fri, 5 Jul 2019 09:51:52 +0200
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On 7/5/19 9:44 AM, Stefan Hajnoczi wrote:
> On Fri, Jul 05, 2019 at 09:12:37AM +0200, Hannes Reinecke wrote:
>> On 7/4/19 3:19 PM, Paolo Bonzini wrote:
>>> On 19/06/19 12:31, Paolo Bonzini wrote:
>>>>> I'm a bit unsure if 'bd->last' is always set; it's quite obvious that
>>>>> it's present if set, but what about requests with 'bd->last == false' ?
>>>>> Is there a guarantee that they will _always_ be followed with a request
>>>>> with bd->last == true?
>>>>> And if so, is there a guarantee that this request is part of the same batch?
>>>> It's complicated.  A request with bd->last == false _will_ always be
>>>> followed by a request with bd->last == true in the same batch.  However,
>>>> due to e.g. errors it may be possible that the last request is not sent.
>>>>  In that case, the block layer sends commit_rqs, as documented in the
>>>> comment above, to flush the requests that have been sent already.
>>>>
>>>> So, a driver that obeys bd->last (or SCMD_LAST) but does not implement
>>>> commit_rqs is bound to have bugs, which is why this patch was not split
>>>> further.
>>>>
>>>> Makes sense?
>>>
>>> Hannes, can you provide your Reviewed-by?
>>>
>> Well ... since you asked for it:
>>
>> Where is the 'commit_rqs' callback actually used?
>> I seem to be going blind, but I can't find it; should be somewhere in
>> the first patch, no?
>> As per description:
>>
>>  * The commit_rqs function is used to trigger a hardware
>>  * doorbell after some requests have been queued with
>>  * queuecommand, when an error is encountered before sending
>>  * the request with SCMD_LAST set.
>>
>> So it should be somewhere in the error path, probably scsi_error or
>> something. But I don't seem to be able to find it ...
> 
> The block layer calls scsi_mq_ops->commit_rqs() from
> blk_mq_dispatch_rq_list() and blk_mq_try_issue_list_directly().
> 
Ah, right.

Now, then:

Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

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