Re: [PATCH] blkif: reconcile protocol specification with in-use implementations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.09.2024 16:19, 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:
> 
>  * 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 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 (even when 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: 67e1c050e36b ('public/io/blkif.h: try to fix the semantics of sector 
> based quantities')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Following the earlier discussion, I was kind of hoping that there would be
at least an outline of some plan here as to (efficiently) dealing with 4k-
sector disks. In the absence of that I'm afraid it is a little harder to
judge whether the proposal here is the best we can do at this point.

Jan

> --- a/xen/include/public/io/blkif.h
> +++ b/xen/include/public/io/blkif.h
> @@ -240,10 +240,6 @@
>   *      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">
> @@ -254,8 +250,8 @@
>   * 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 integer
>   *      multiple of "physical-sector-size", if that node is present.
>   *
>   
> *****************************************************************************
> @@ -338,6 +334,7 @@
>   * feature-large-sector-size
>   *      Values:         0/1 (boolean)
>   *      Default Value:  0
> + *      Notes:          DEPRECATED, 12
>   *
>   *      A value of "1" indicates that the frontend will correctly supply and
>   *      interpret all sector-based quantities in terms of the "sector-size"
> @@ -411,6 +408,11 @@
>   *(10) The discard-secure property may be present and will be set to 1 if the
>   *     backing device supports secure discard.
>   *(11) Only used by Linux and NetBSD.
> + *(12) Possibly only ever implemented by the QEMU Qdisk backend and the 
> Windows
> + *     PV block frontend.  Other backends and frontends supported 
> 'sector-size'
> + *     values greater than 512 before such feature was added.  Frontends 
> should
> + *     not expose this node, neither should backends make any decisions based
> + *     on it being exposed by the frontend.
>   */
>  
>  /*
> @@ -621,9 +623,12 @@
>  /*
>   * 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.
> + * blkif_request_indirect are all in units of 512 bytes, regardless of 
> whether the
> + * 'sector-size' xenstore node contains a value greater than 512.
> + *
> + * However the value in those fields must be properly aligned to the logical
> + * sector size reported by the 'sector-size' xenstore node, see 'Backend 
> Device
> + * Properties' section.
>   */
>  struct blkif_request_segment {
>      grant_ref_t gref;        /* reference to I/O buffer frame        */