On 10.09.24 15:59, Roger Pau Monné wrote:
On Tue, Sep 10, 2024 at 03:46:00PM +0200, Jürgen Groß wrote:On 10.09.24 13:46, Roger Pau Monne wrote:Current blkif implementations (both backends and frontends) have all slight differences about how they handle the 'sector-size' xenstore node, and how other fields are derived from this value or hardcoded to be expressed in units of 512 bytes.To give some context, this is an excerpt of how different implementations use the value in 'sector-size' as the base unit for to other fields rather than just to set the logical sector size of the block device: │ sectors xenbus node │ requests sector_number │ requests {first,last}_sect ────────────────────────┼─────────────────────┼────────────────────────┼─────────────────────────── FreeBSD blk{front,back} │ sector-size │ sector-size │ 512 ────────────────────────┼─────────────────────┼────────────────────────┼─────────────────────────── Linux blk{front,back} │ 512 │ 512 │ 512 ────────────────────────┼─────────────────────┼────────────────────────┼─────────────────────────── QEMU blkback │ sector-size │ sector-size │ sector-size ────────────────────────┼─────────────────────┼────────────────────────┼─────────────────────────── Windows blkfront │ sector-size │ sector-size │ sector-size ────────────────────────┼─────────────────────┼────────────────────────┼─────────────────────────── MiniOS │ sector-size │ 512 │ 512 An attempt was made by 67e1c050e36b in order to change the base units of the request fields and the xenstore 'sectors' node. That however only lead to more confusion, as the specification now clearly diverged from the reference implementation in Linux. Such change was only implemented for QEMU Qdisk and Windows PV blkfront. Partially revert to the state before 67e1c050e36b while adjusting the documentation for 'sectors' to match what it used to be previous to 2fa701e5346d: * Declare 'feature-large-sector-size' deprecated. Frontends should not expose the node, backends should not make decisions based on its presence. * Clarify that 'sectors' xenstore node and the requests fields are always in 512-byte units, like it was previous to 2fa701e5346d and 67e1c050e36b. All base units for the fields used in the protocol are 512-byte based, the xenbus 'sector-size' field is only used to signal the logic block size. When 'sector-size' is greater than 512, blkfront implementations must make sure that the offsets and sizes (despite being expressed in 512-byte units) are aligned to the logical block size specified in 'sector-size', otherwise the backend will fail to process the requests. This will require changes to some of the frontends and backends in order to properly support 'sector-size' nodes greater than 512. Fixes: 2fa701e5346d ('blkif.h: Provide more complete documentation of the blkif interface') Fixes: 67e1c050e36b ('public/io/blkif.h: try to fix the semantics of sector based quantities') Signed-off-by: Roger Pau Monné <roger....@citrix.com> --- Changes since v1: - Update commit message. - Expand comments. --- xen/include/public/io/blkif.h | 50 ++++++++++++++++++++++++++--------- 1 file changed, 38 insertions(+), 12 deletions(-) diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h index 22f1eef0c0ca..da893eb379db 100644 --- a/xen/include/public/io/blkif.h +++ b/xen/include/public/io/blkif.h @@ -237,12 +237,16 @@ * sector-size * Values: <uint32_t> * - * The logical block size, in bytes, of the underlying storage. This - * must be a power of two with a minimum value of 512. + * The logical block size, in bytes, of the underlying storage. This must + * be a power of two with a minimum value of 512. The sector size should + * only be used for request segment length and alignment. * - * 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). + * When exposing a device that uses 4096 logical sector sizes, the onlys/uses 4096 logical sector sizes/uses a logical sector size of 4096/Yes, that's better.+ * difference xenstore wise will be that 'sector-size' (and possibly + * 'physical-sector-size' if supported by the backend) will be 4096, but + * the 'sectors' node will still be calculated using 512 byte units. The + * sector base units in the ring requests fields will all be 512 byte + * based despite the logical sector size exposed in 'sector-size'. * * physical-sector-size * Values: <uint32_t> @@ -254,8 +258,8 @@ * sectors * Values: <uint64_t> * - * The size of the backend device, expressed in units of "sector-size". - * The product of "sector-size" and "sectors" must also be an integer + * The size of the backend device, expressed in units of 512b. + * The product of "sector-size" * 512 must also be an integerDYM: The product of "sectors" * 512 must also be an integer ... ?Indeed.
With those 2 changes applied you can add my: Reviewed-by: Juergen Gross <jgr...@suse.com> Juergen
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