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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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