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

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Message-ID: <20130305214641.GD8235@phenom.dumpdata.com>
Date:	Tue, 5 Mar 2013 16:46:41 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	james.harper@...digoit.com.au
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 Tue, Mar 05, 2013 at 06:07:57PM +0100, Roger Pau Monné wrote:
> 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...

I would think that most OSes use the same framework. The ones that
are of notable interest are the Windows and BSD. Lets CC James here

> 
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