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Subject: Re: [PATCH 1/2] scsi_host: add support for request batching
From:   Paolo Bonzini <pbonzini@redhat.com>
To:     Hannes Reinecke <hare@suse.de>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     jejb@linux.ibm.com, martin.petersen@oracle.com,
        linux-scsi@vger.kernel.org, stefanha@redhat.com
References: <20190530112811.3066-1-pbonzini@redhat.com>
 <20190530112811.3066-2-pbonzini@redhat.com>
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 <aaa344bf-af29-0485-4e83-5442331a2c9c@redhat.com>
Message-ID: <afea12a1-47b3-0ebe-a3c2-6adc615bbddf@redhat.com>
Date:   Thu, 4 Jul 2019 15:19:08 +0200
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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?

Thanks,

Paolo