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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/09/2024 1:50 pm, Roger Pau Monné wrote:
> On Thu, Sep 12, 2024 at 02:06:23PM +0200, Jan Beulich wrote:
>> 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?
> I would suggest to backport to open release branches, as people
> working on protocol implementations might not use the headers from
> unstable, but rather from the last release.

Independently to backport, this definitely warrants a CHANGELOG entry.

We need to take all reasonable steps to make people aware that there are
breakages here.

~Andrew