Re: [Xen-devel] [PATCH] public/io/blkif.h: make the comments on "sectors" self-consistent

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> Sent: 20 March 2019 13:53
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] public/io/blkif.h: make the comments on "sectors" 
> self-consistent
> 
> On Wed, Mar 20, 2019 at 12:52:28PM +0000, Paul Durrant wrote:
> > Currently the comment at line #267 claims that the value should be
> > expressed in number logical sectors, whereas the comment at line #613
> > states that the value should be expressed strictly in units of 512 bytes.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > ---
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >
> > Looking at xen-blkfront in Linux, I'm also not convinced that it would
> > function correctly is sector-size != 512 anyway so I wonder whether this
> > patch should go further and define that sector-size is strictly 512.
> 
> I thought it actually did work with a CD-ROM backed ISO using blkfront?
> 

I've not tried that. What worries me is the call to xlvbd_alloc_gendisk() which 
takes sectors as an argument and passes it through to set_capacity() without 
scaling with sector-size in any way, which is would presumably need to do is 
sector-size != 512 (if we believe that sectors should be in terms of 512 byte 
units).

  Paul

> > ---
> >  xen/include/public/io/blkif.h | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
> > index 15a71e3fea..d7c904d9dc 100644
> > --- a/xen/include/public/io/blkif.h
> > +++ b/xen/include/public/io/blkif.h
> > @@ -264,8 +264,7 @@
> >   * sectors
> >   *      Values:         <uint64_t>
> >   *
> > - *      The size of the backend device, expressed in units of its logical
> > - *      sector size ("sector-size").
> > + *      The size of the backend device, expressed in units of 512 bytes.
> >   *
> >   
> > *****************************************************************************
> >   *                            Frontend XenBus Nodes
> > --
> > 2.20.1.2.gb21ebb671
> >

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