Re: [PATCH v3] blkif: reconcile protocol specification with in-use implementations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.09.2024 11:57, 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.pau@xxxxxxxxxx>
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

The Fixes: tags generally suggest this wants backporting. I'm a little uncertain
here though, as it won't really affect anything that is built. Opinions?

Jan