@@ -254,18 +254,26 @@
* sector-size
* Values: <uint32_t>
*
- * The logical sector size, in bytes, of the backend device.
+ * The logical block size, in bytes, of the underlying storage. This
+ * must be a power of two with a minimum value of 512.
+ *
+ * NOTE: Because of implementation bugs in some frontends this must be
+ * set to 512, unless the frontend advertizes a non-zero value
+ * in its "feature-large-sector-size" xenbus node. (See below).
*
* physical-sector-size
* Values: <uint32_t>
+ * Default Value: <"sector-size">
*
- * The physical sector size, in bytes, of the backend device.
+ * The physical block size, in bytes, of the backend storage. This
+ * must be an integer multiple of "sector-size".
*
* sectors
* Values: <uint64_t>
*
- * The size of the backend device, expressed in units of its logical
- * sector size ("sector-size").
+ * The size of the backend device, expressed in units of "sector-size".
+ * The product of "sector-size" and "sectors" must also be an integer
+ * multiple of "physical-sector-size", if that node is present.
*
*****************************************************************************
* Frontend XenBus Nodes
@@ -321,6 +329,8 @@
* The size of the frontend allocated request ring buffer in units of
* machine pages. The value must be a power of 2.
*
+ *--------------------------------- Features ---------------------------------
+ *
* feature-persistent
* Values: 0/1 (boolean)
* Default Value: 0
@@ -342,6 +352,17 @@
* decides to limit the maximum number of persistently mapped grants
* to a value less than RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST.
*
+ * feature-large-sector-size
+ * Values: 0/1 (boolean)
+ * Default Value: 0
+ *
+ * A value of "1" indicates that the frontend will correctly supply and
+ * interpret all sector-based quantities in terms of the "sector-size"
+ * value supplied in the backend info, whatever that may be set to.
+ * If this node is not present or its value is "0" then it is assumed
+ * that the frontend requires that the logical block size is 512 as it
+ * is hardcoded (which is the case in some frontend implementations).
+ *
*------------------------- Virtual Device Properties -------------------------
*
* device-type
@@ -607,12 +628,11 @@
#define BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST 8
/*
- * NB. first_sect and last_sect in blkif_request_segment, as well as
- * sector_number in blkif_request, are always expressed in 512-byte units.
- * However they must be properly aligned to the real sector size of the
- * physical disk, which is reported in the "physical-sector-size" node in
- * the backend xenbus info. Also the xenbus "sectors" node is expressed in
- * 512-byte units.
+ * NB. 'first_sect' and 'last_sect' in blkif_request_segment, as well as
+ * 'sector_number' in blkif_request, blkif_request_discard and
+ * blkif_request_indirect are sector-based quantities. See the description
+ * of the "feature-large-sector-size" frontend xenbus node above for
+ * more information.
*/
struct blkif_request_segment {
grant_ref_t gref; /* reference to I/O buffer frame */
The semantics of sector based quantities, such as first_sect and last_sect in blkif_request_segment, and the value of "sectors" in the backend info in xenstore have become confused. Some comments in the header suggest they should be supplied/interpreted strictly in terms of 512-byte units, others suggest they should be scaled by the value of "sector-size" i.e. the logical block size of the underlying backend storage. This confusion has caused mixed semantics to become ingrained in frontend implementations. For instance Linux xen-blkfront.c contains code such as: fsect = offset >> 9; lsect = fsect + (len >> 9) - 1; whereas the Windows XENVBD frontend contains the following equivalent code: Segment->FirstSector = (UCHAR)((Offset + SectorSize - 1) / SectorSize); *SectorsNow = __min(SectorsLeft, SectorsPerPage - Segment->FirstSector); Segment->LastSector = (UCHAR)(Segment->FirstSector + *SectorsNow - 1); (where SectorSize is the "sector-size" value advertized in xenstore). Thus it has become unsafe for a backend to set "sector-size" to anything other than 512 as it does not know which way the frontend is coded. This patch is intended to clarify the situation and also introduce a mechanism to allow logical block sizes of more than 512 to be supported... A new frontend feature node is specified: 'feature-large-sector-size'. If this node is present and set to "1" then it means that frontend is coded to supply and interpret all sector based quantities in terms of the the advertized "sector-size" value rather than a hardcoded size of 512. Signed-off-by: Paul Durrant <paul.durrant@citrix.com> Reviewed-by: Juergen Gross <jgross@suse.com> --- Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Roger Pau Monné <roger.pau@citrix.com> Cc: Athony PERARD <anthony.perard@citrix.com> Cc: Andrew Cooper <andrew.cooper3@citrix.com> Cc: George Dunlap <George.Dunlap@eu.citrix.com> Cc: Ian Jackson <ian.jackson@eu.citrix.com> Cc: Jan Beulich <jbeulich@suse.com> Cc: Julien Grall <julien.grall@arm.com> Cc: Stefano Stabellini <sstabellini@kernel.org> Cc: Tim Deegan <tim@xen.org> Cc: Wei Liu <wei.liu2@citrix.com> This is actually v3 of the patch posted in [1]. Please see the thread starting there for more context... v3: - Fix commit comment v2: - Drop Konrad's original ack since the patch has substantially changed - Cc several different OS blkfront maintainers and 'the rest' maintainers for more opinion [1] https://lists.xenproject.org/archives/html/xen-devel/2019-03/msg01600.html --- xen/include/public/io/blkif.h | 40 ++++++++++++++++++++++++++--------- 1 file changed, 30 insertions(+), 10 deletions(-)