Re: [PATCH v2] blkif: reconcile protocol specification with in-use implementations

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

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.pau@xxxxxxxxxx>
> > ---
> > 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 
> > only
> 
> s/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 integer
> 
> DYM: The product of "sectors" * 512 must also be an integer ... ?

Indeed.

Thanks, Roger.