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Re: [Xen-devel] [PATCH 0 of 5] blkif.h: Document protocol and existing extensions

>>> On 03.02.12 at 06:24, "Justin T. Gibbs" <justing@xxxxxxxxxxxxxxxx> wrote:
> This patch series attempts to document the blkif PV interface and
> the various extensions to it that are out in the wild.  I assembled
> this information by reviewing xend, various versions of the Linux
> blkif drivers, fixing bugs in the FreeBSD blkfront driver, and
> writing and testing a blkback driver for FreeBSD.

Thanks for doing this!

> All but the last patch in the series is source compatible with
> existing drivers.  This final patch adds FreeBSD's extension to
> allow an I/O to span multiple ring entries.  The number of outstanding
> requests, and the max I/O size and number of segments per request,
> can all be negotiated.  Drivers offering this extension are backwards
> compatible with existing drivers, but the definition of
> BLKIF_MAX_SEGMENTS_PER_REQUEST has been changed to reflect the new
> limit of 255.  A global replacement of BLKIF_MAX_SEGMENTS_PER_REQUEST
> with BLKIF_MAX_SEGMENTS_PER_HEADER_BLOCK is all that's required to
> adjust drivers without the extension to this header change.

No, this sort of change is just not valid to be done. Changes to
public headers must always (default to) be backwards compatible.


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