Re: [Xen-devel] [PATCH v4 06/14] xen: Move the hvm_start_info C representation from libxc to public/xen.h

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Mar 15, 2016 at 02:09:46AM -0600, Jan Beulich wrote:
> >>> On 14.03.16 at 18:55, <anthony.perard@xxxxxxxxxx> wrote:
> > --- a/xen/include/public/xen.h
> > +++ b/xen/include/public/xen.h
> > @@ -841,6 +841,37 @@ typedef struct start_info start_info_t;
> >   */
> >  #define XEN_HVM_START_MAGIC_VALUE 0x336ec578
> >  
> > +#if defined(__i386__) || defined(__x86_64__)
> > +/* C representation of the x86/HVM start info layout.
> > + *
> > + * The canonical definition of this layout is abrove, this is just a way to
> > + * represent the layout described there using C types.
> > + *
> > + * NB: the packed attribute is not really needed, but it helps us enforce
> > + * the fact this this is just a representation, and it might indeed
> > + * be required in the future if there are alignment changes.
> > + */
> > +struct hvm_start_info {
> > +    uint32_t magic;             /* Contains the magic value 0x336ec578     
> >   */
> > +                                /* ("xEn3" with the 0x80 bit of the "E" 
> > set).*/
> > +    uint32_t version;           /* Version of this structure.              
> >   */
> > +    uint32_t flags;             /* SIF_xxx flags.                          
> >   */
> > +    uint32_t cmdline_paddr;     /* Physical address of the command line.   
> >   */
> > +    uint32_t nr_modules;        /* Number of modules passed to the kernel. 
> >   */
> > +    uint32_t modlist_paddr;     /* Physical address of an array of         
> >   */
> > +                                /* hvm_modlist_entry.                      
> >   */
> > +    uint32_t rsdp_paddr;        /* Physical address of the RSDP ACPI data  
> >   */
> > +                                /* structure.                              
> >   */
> > +} __attribute__((packed));
> > +
> > +struct hvm_modlist_entry {
> > +    uint64_t paddr;             /* Physical address of the module.         
> >   */
> > +    uint64_t size;              /* Size of the module in bytes.            
> >   */
> > +    uint64_t cmdline_paddr;     /* Physical address of the command line.   
> >   */
> > +    uint64_t reserved;
> > +} __attribute__((packed));
> > +#endif /* x86 */
> 
> This needs extra care: Either the packed attributes need to be
> dropped (Roger - why did they get put there in the first place?),

Presumarily b/c on 32-bit and 64-bit the size of the structure would
be different otherwise...

> or their use needs to be shielded from compilers not understanding
> them.

.. which begs the question - How come we didn't make the structure 64-bit nice?
As in add another uint32_t to it? And then we can ditch the packed?



> 
> Jan
> 
> 
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