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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?). 

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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