Re: [Xen-devel] [PATCH v2 0/2] xen-block: fix sector size confusion

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper
> Sent: 27 March 2019 18:20
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; 
> qemu-block@xxxxxxxxxx;
> qemu-devel@xxxxxxxxxx
> Cc: Kevin Wolf <kwolf@xxxxxxxxxx>; Stefano Stabellini 
> <sstabellini@xxxxxxxxxx>; Max Reitz
> <mreitz@xxxxxxxxxx>; Stefan Hajnoczi <stefanha@xxxxxxxxxx>; Anthony Perard 
> <anthony.perard@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2 0/2] xen-block: fix sector size confusion
> 
> On 27/03/2019 17:32, Paul Durrant wrote:
> > The Xen blkif protocol is confusing but discussion with the maintainer
> > has clarified that sector based quantities in requests and the 'sectors'
> > value advertized in xenstore should always be in terms of 512-byte
> > units and not the advertised logical 'sector-size' value.
> >
> > This series fixes xen-block to adhere to the spec.
> 
> I thought we agreed that hardcoding things to 512 bytes was the wrong
> thing to do.

To some extent we decided it was the *only* thing to do.

> 
> I was expecting something like:
> 
> 1) Clarify the spec with the intended meaning, (which is what some
> implementations actually use already) and wont cripple 4k datapaths.
> 2) Introduce a compatibility key for "I don't rely on sector-size being
> 512", which fixed implementations should advertise.
> 3) Specify that because of bugs in the spec which got out into the wild,
> drivers which don't find the key being advertised by the other end
> should emulate sector-size=512 for compatibility with broken
> implementations.

Yes, that's how we are going to fix things.

> 
> Whatever the eventual way out, the first thing which needs to happen is
> an update to the spec, before actions are taken to alter existing
> implementations.

Well the implementation is currently wrong w.r.t. the spec and these patches 
fix that. As long as sector-size remains at 512 then no existing frontend 
should break, so I guess you could argue that patch #2 should also make sure 
that sector-size is also 512... but that is not yet in the spec.
I guess I'm ok to defer patch #2 until a revised spec. is agreed, but the ship 
has already sailed as far as patch #1 goes.

Anthony, thoughts?

  Paul 

> 
> ~Andrew
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