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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Sep 04, 2024 at 11:31:08AM +0200, Jan Beulich wrote:
> On 04.09.2024 10:21, Roger Pau Monné wrote:
> > On Tue, Sep 03, 2024 at 04:36:37PM +0200, Jan Beulich wrote:
> >> 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@citrix.com>
> >>
> >> 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.
> > 
> > What do you mean with efficiently?
> > 
> > 4K disks will set 'sector-size' to 4096, so the segments setup by the
> > frontends in the requests will all be 4K aligned (both address and
> > size).
> 
> Will they, despite granularity then being 512b?

The added text to blkif.h states:

"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."

'those fields' in the text above refers to the sector based offsets
and sizes in blkif_request & other ring structs.  So while the base
units of the fields are 512-byte based, the resulting offsets and
sizes should be aligned to the value in 'sector-size'.

> Perhaps I misunderstood the proposal then, and you're retaining the
> ability to have "sector-size" != 512, just that any I/O done is not
> supposed to consider that setting.

No, I/O is supposed to consider that setting, is just that the base
unit in the ring structures will always be 512-byte based, regardless
of what 'sector-size' contains.

> I guess I mis-read the 2nd to last
> paragraph of the description; I'm sorry. "even when expressed in 512-
> byte units" reads to me as if other units are permissible. Maybe it
> was really meant to be "despite being expressed in 512-byte units"?

Sure, I will adjust the commit message.

Thanks, Roger.