[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] blkif: reconcile protocol specification with in-use implementations
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@xxxxxxxxxx> >> >> 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? 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. 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"? Jan
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