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Re: [Xen-devel] [PATCH v2] public/io/blkif.h: try to fix the semantics of sector based quantities
- To: Paul Durrant <paul.durrant@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Wed, 3 Apr 2019 15:27:48 +0200
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Athony PERARD <anthony.perard@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 03 Apr 2019 13:28:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 02/04/2019 18:58, Paul Durrant wrote:
> This is actually v2 of the patch posted in [1]. Please see the thread
> starting there for more context...
>
> The semantics of sector based quanties, such as first_sect and last_sect in
> blkif_request_segment, and the value of "sectors" in the backend info
> in xenstore have become confused. Some comments in the header suggest they
> should be supplied/interpreted strictly in terms of 512-byte units, others
> suggest they should be scaled by the value of "sector-size" i.e. the
> logical block size of the underlying backend storage.
> This confusion has caused mixed semantics to become ingrained in frontend
> implementations. For instance Linux xen-blkfront.c contains code such as:
>
> fsect = offset >> 9;
> lsect = fsect + (len >> 9) - 1;
>
> whereas the Windows XENVBD frontend contains the following equivalent code:
>
> Segment->FirstSector = (UCHAR)((Offset + SectorSize - 1) / SectorSize);
> *SectorsNow = __min(SectorsLeft, SectorsPerPage - Segment->FirstSector);
> Segment->LastSector = (UCHAR)(Segment->FirstSector + *SectorsNow - 1);
>
> (where SectorSize is the "sector-size" value advertized in xenstore).
>
> Thus it has become unsafe for a backend to set "sector-size" to anything
> other than 512 as it does not know which way the frontend is coded.
>
> This patch is intended to clarify the situation and also introduce a
> mechanism to allow logical block sizes of more than 512 to be supported...
>
> A new frontend feature node is specified: 'feature-large-sector-size'.
> If this node is present and set to "1" then it means that frontend is
> coded to supply and interpret all sector based quantities in terms of the
> the advertized "sector-size" value rather than a hardcoded size of 512.
>
> [1] https://lists.xenproject.org/archives/html/xen-devel/2019-03/msg01600.html
>
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Apart from the already mentioned commit message issues:
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Juergen
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