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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-11-13 at 11:07 +0000, Paul Durrant wrote:
> > -----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.

I am talking about the general case for all xen/include/public headers,
not these structs specifically.

There should be a well specified default for the struct layout. If
particular structs diverge from this (and being consistent across 32-
and 64-bit is a good reason to do so) then suitable padding and perhaps
#ifdefs might be needed.

Ian.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel