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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Thanks, Roger.