linux-kernel - Re: [PATCH RFC 12/12] xen-block: implement indirect descriptors

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Message-ID: <5136266D.8050707@citrix.com>
Date:	Tue, 5 Mar 2013 18:07:57 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH RFC 12/12] xen-block: implement indirect descriptors

On 04/03/13 21:41, Konrad Rzeszutek Wilk wrote:
> On Thu, Feb 28, 2013 at 11:28:55AM +0100, Roger Pau Monne wrote:
>> Indirect descriptors introduce a new block operation
>> (BLKIF_OP_INDIRECT) that passes grant references instead of segments
>> in the request. This grant references are filled with arrays of
>> blkif_request_segment_aligned, this way we can send more segments in a
>> request.
>>
>> The proposed implementation sets the maximum number of indirect grefs
>> (frames filled with blkif_request_segment_aligned) to 256 in the
>> backend and 64 in the frontend. The value in the frontend has been
>> chosen experimentally, and the backend value has been set to a sane
>> value that allows expanding the maximum number of indirect descriptors
>> in the frontend if needed.
> 
> So we are still using a similar format of the form:
> 
> <gref, first_sec, last_sect, pad>, etc.
> 
> Why not utilize a layout that fits with the bio sg? That way
> we might not even have to do the bio_alloc call and instead can
> setup an bio (and bio-list) with the appropiate offsets/list?
> 
> Meaning that the format of the indirect descriptors is:
> 
> <gref, offset, next_index, pad>
> 
> We already know what the first_sec and last_sect are - they
> are basically: sector_number +  nr_segments * (whatever the sector size is) + offset

This will of course be suitable for Linux, but what about other OSes, I
know they support the traditional first_sec, last_sect (because it's
already implemented), but I don't know how much work will it be for them
to adopt this. If we have to do such a change I will have to check first
that other frontend/backend can handle this easily also, I wouldn't like
to simplify this for Linux by making it more difficult to implement in
other OSes...

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