Re: [Xen-devel] [PATCH 4/5] xen: Enforce casting for guest_handle_cast

[Jean Guyader <jean.guyader@xxxxxxxxxx>]

On 14/06 05:00, Jan Beulich wrote:
> >>> On 14.06.12 at 17:39, Jean Guyader <jean.guyader@xxxxxxxxxx> wrote:
> > Here are the structs:
> > 
> > typedef struct v4v_ring_data_ent                                            
> >  
> > {                                                                           
> >  
> >     struct v4v_addr ring;                                                   
> >  
> >     uint16_t flags;                                                         
> >  
> >     uint16_t pad0;                                                          
> >  
> >     uint32_t space_required;                                                
> >  
> >     uint32_t max_message_size;                                              
> >  
> > } v4v_ring_data_ent_t;                                                      
> >  
> > DEFINE_XEN_GUEST_HANDLE (v4v_ring_data_ent_t);                              
> >  
> >                                                                             
> >  
> > typedef struct v4v_ring_data                                                
> >  
> > {                                                                           
> >  
> >     uint64_t magic;                                                         
> >  
> >     uint32_t nent;                                                          
> >  
> >     uint32_t padding;                                                       
> >  
> >     uint64_t reserved[4];                                                   
> >  
> >     v4v_ring_data_ent_t ring[0];                                            
> >  
> > } v4v_ring_data_t;                                                          
> >  
> > DEFINE_XEN_GUEST_HANDLE (v4v_ring_data_t);
> > 
> > I get a XEN_GUEST_HANDLE(v4v_ring_data_t) as argument of my hypercall and I 
> > would like to access the ring data inside it which is a XEN_GUEST_HANDLE as 
> > well.
> > 
> > Here is the code that I use for doing that (with explicte cast in 
> > guest_handle_cast):
> >     XEN_GUEST_HANDLE (v4v_ring_data_ent_t) ring_data_ent_hnd;
> >     XEN_GUEST_HANDLE (uint8_t) slop_hnd =
> >         guest_handle_cast (ring_data_hnd, uint8_t);
> >     guest_handle_add_offset (slop_hnd, sizeof (v4v_ring_data_t));
> >     ring_data_ent_hnd =
> >         guest_handle_cast (slop_hnd, v4v_ring_data_ent_t);
> >     ret = v4v_fill_ring_datas (d, ring_data.nent, ring_data_ent_hnd);
> 
> So you really don't want to add anything (as not being type-safe),
> but instead want to get a guest handle representation for accessing
> the ring[0] member. That is, I'd introduce a guest_handle_for_field()
> for this purpose. Let me see whether I can put something together
> for you early next week.
> 

Thanks but I'll follow Tim's advice and get rid of the wrapper
structure. I still think having some macro to get a handle from
a field can be quite useful in general.

Jean

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