Re: [Xen-devel] [PATCH v3] public/io/blkif.h: try to fix the semantics of sector based quantities

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Apr 04, 2019 at 12:40:02PM +0100, Paul Durrant wrote:
> The semantics of sector based quantities, 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.
> 
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

I think Juergen's Rb is sufficient.

I will commit this patch today.

Wei.

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