(+ Juergen as he maintains the PV I/O with Konrad) On 02/04/2019 17:58, Paul Durrant wrote:
This is actually v2 of the patch posted in [1]. Please see the thread starting there for more context...
Should not this belong after ---?
The semantics of sector based quanties, such as first_sect and last_sect in
s/quanties/quantities/
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. [1] https://lists.xenproject.org/archives/html/xen-devel/2019-03/msg01600.html Signed-off-by: Paul Durrant <paul.durr...@citrix.com> --- Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com> Cc: Roger Pau Monné <roger....@citrix.com> Cc: Athony PERARD <anthony.per...@citrix.com> Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: George Dunlap <george.dun...@eu.citrix.com> Cc: Ian Jackson <ian.jack...@eu.citrix.com> Cc: Jan Beulich <jbeul...@suse.com> Cc: Julien Grall <julien.gr...@arm.com> Cc: Stefano Stabellini <sstabell...@kernel.org> Cc: Tim Deegan <t...@xen.org> Cc: Wei Liu <wei.l...@citrix.com> 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 --- xen/include/public/io/blkif.h | 40 ++++++++++++++++++++++++++--------- 1 file changed, 30 insertions(+), 10 deletions(-) diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h index 15a71e3fea..d4a34de94c 100644 --- a/xen/include/public/io/blkif.h +++ b/xen/include/public/io/blkif.h @@ -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 */
-- Julien Grall _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
