Re: [Xen-devel] [PATCH] blkif: add indirect descriptors interface to public headers

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell
> Sent: 12 November 2013 14:29
> To: Konrad Rzeszutek Wilk
> Cc: Paul Durrant; xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org); Jan Beulich;
> Roger Pau Monne
> Subject: Re: [Xen-devel] [PATCH] blkif: add indirect descriptors interface to
> public headers
> 
> On Tue, 2013-11-12 at 09:22 -0500, Konrad Rzeszutek Wilk wrote:
> 
> > > > +struct blkif_request_indirect {
> > > > +    uint8_t        operation;    /* BLKIF_OP_INDIRECT                  
> > > >   */
> > > > +    uint8_t        indirect_op;  /* BLKIF_OP_{READ/WRITE}              
> > > >   */
> > > > +    uint16_t       nr_segments;  /* number of segments                 
> > > >   */
> > >
> > > This is going to be a problem. What alignment boundary are you
> > expecting the next field to start on? AFAIK 32-bit gcc will 4-byte
> > align it, 32-bit MSVC will 8-byte align it.
> > >
> >
> > Oh no. I thought that the Linux one had this set correctly, ah it did:
> >
> >
> > struct blkif_request_indirect {
> > [...]
> > } __attribute__((__packed__));
> 
> That attribute packed isn't allowed in the public interface headers.
> 
> Since compilers do differ in their packing, and guests may be using
> various pragmas, it might be useful to write down that for x86 these
> headers are to be treated as using the <WHATEVER> ABI (gcc? Some Intel
> standard?).
> 

Can we go for types aligned on their size then rather than gcc brokenness.

  Paul

> For ARM we reference the specific standard[0]. It is up to the guest OS
> to make sure that it's version of the headers lay things out following
> that standard (NB Linux blkif.h is currently buggy on ARM in this
> regard, Julien has the details).
> 
> Ian.
> 
> [0]
> http://xenbits.xen.org/docs/unstable/hypercall/arm/include,public,arch-
> arm.h.html#incontents_arm_abi

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