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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell
> Sent: 13 November 2013 09:27
> To: Paul Durrant
> Cc: Konrad Rzeszutek Wilk; 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 15:16 +0000, Paul Durrant wrote:
> > > -----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.
> 
> We should go for some existing well defined ABI spec not make up our
> own.
> 
> In effect the x86 ABI has historically been de-facto specified as the
> gcc ABI.
> 

Since the linux headers seem to hardcode the x64 ABI for this struct, do we 
need to support an x86 variant? After all there's no backwards compatibility 
issue here.

   Paul

> >
> >   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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