Re: [PATCH 2/3] xenoprof: Handle 32-bit guest stacks properly in a 64-bit hypervisor

[Marcus Granado <marcus.granado@citrix.com>]

On 23/01/12 09:50, Jan Beulich wrote:
> If you're adding a compat mode guest case here, then you should
> also use compat mode accessors (compat_handle_okay(),
> __copy_from_compat_offset()), implying that you also have a local
> handle variable of the appropriate type (and perhaps moving the
> native one down into the 'else' body).

I'm trying to understand the compat handle. It is not clear to me how to 
map one from head (a 64-bit pointer), since COMPAT_HANDLE seems to store 
a 32-bit compat_ptr_t value in its structure. Ideally, what I would like 
to do is

COMPAT_HANDLE(char) guest_head = map_guest_handle_to_compat_handle 
(guest_handle_from_ptr(head, char));
or
COMPAT_HANDLE(char) guest_head = compat_handle_from_ptr(head, char));
but I can't find any equivalent functions in any header.

The following line compiles,
COMPAT_HANDLE(char) guest_head = { (full_ptr_t)head };
but it looks like, in this case, the compat handle structure in compat.h 
will truncate the most significant bits from the head pointer, so 
compat_handle_okay(guest_head,...) and 
__copy_from_compat(...,guest_head,...) below will be using a truncated 
pointer:

      56 static struct frame_head *
      57 dump_guest_backtrace(struct domain *d, struct vcpu *vcpu,
      58                      struct frame_head * head, int mode)
      59 {
      60     struct frame_head bufhead[2];
      61
      62 #ifdef CONFIG_X86_64
      63     if ( is_32bit_vcpu(vcpu) )
      64     {
      65         COMPAT_HANDLE(char) guest_head = { (full_ptr_t)head };
      66         struct frame_head_32bit bufhead32[2];
      67         /* Also check accessibility of one struct frame_head 
beyond */
      68         if (!compat_handle_okay(guest_head, sizeof(bufhead32)))
      69             return 0;
      70         if (__copy_from_compat((char *)bufhead32, guest_head,
      71                                      sizeof(bufhead32)))
      72             return 0;
      73         bufhead[0].ebp=(struct frame_head 
*)(full_ptr_t)bufhead32[0].ebp;
      74         bufhead[0].ret=bufhead32[0].ret;
      75     }
      76     else
      77 #endif

Any advice? Maybe the best option in this case is to avoid the compat* 
functions and to use the original guest* functions instead.

Thanks,
Marcus