[PATCH] fixes to xen-blk[back|front] to deal with 32/64 host/guest combination with BLKIF_DISCARD (v1).

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

These two patches came out of hitting https://bugzilla.redhat.com/show_bug.cgi?id=824641
where a 32-bit guest would send a BLKIF_DISCARD request to a 64-bit
host. The xen-blkback did not copy the 'id' field into the response (it ended
up with a random value), which the frontend uses to lookup in its array to
find the 'struct request' that corresponded to this BLKIF_DISCARD.

The result was the __blk_end_request_all ended being called with a NULL
pointer and crashed the guest.

The fix is easy enough - make xen-blkback copy the 'id' field when
dealing with 32/64 or 64/32 host/guest combinations. The problem
does not exist if we are using a 64/64 combination.

I also added two BUG_ON to xen-blkfront to detect this misuse of 'id'
field in case there are other backends that do this.

If there are no problems with these patches I was thinking to put them
in my stable/for-jens-3.5 tree and ask Jens to pull them.

 drivers/block/xen-blkback/common.h |    2 ++
 drivers/block/xen-blkfront.c       |    7 +++++++
 2 files changed, 9 insertions(+), 0 deletions(-)