Re: [PATCH v8 4/5] libxc: Provide set_xen_guest_handle_offset macro

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-05-06 at 14:15 -0400, Boris Ostrovsky wrote:
> Add set_xen_guest_handle_offset() macro that can be used for setting
> xen_guest_handle to an offset into hypercall buffer.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> ---
>  tools/libxc/include/xenctrl.h |   11 ++++++++---
>  1 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
> index 8c8eae6..4e6f62c 100644
> --- a/tools/libxc/include/xenctrl.h
> +++ b/tools/libxc/include/xenctrl.h
> @@ -322,16 +322,21 @@ typedef struct xc_hypercall_buffer xc_hypercall_buffer_t;
>   * Set a xen_guest_handle in a type safe manner, ensuring that the
>   * data pointer has been correctly allocated.
>   */
> -#undef set_xen_guest_handle
> -#define set_xen_guest_handle(_hnd, _val)                        \
> +#undef set_xen_guest_handle_offset

This isn't strictly speaking needed since you aren't trying to override
a version of this interface provided by the hypervisor headers.

> +#define set_xen_guest_handle_offset(_hnd, _val, _off)           \
>      do {                                                        \
>          xc_hypercall_buffer_t _hcbuf_hnd1;                      \
>          typeof(XC__HYPERCALL_BUFFER_NAME(_val)) *_hcbuf_hnd2 =  \
>                  HYPERCALL_BUFFER(_val);                         \
>          (void) (&_hcbuf_hnd1 == _hcbuf_hnd2);                   \
> -        set_xen_guest_handle_raw(_hnd, (_hcbuf_hnd2)->hbuf);    \
> +        set_xen_guest_handle_raw(_hnd,                          \
> +                                 (_hcbuf_hnd2)->hbuf + (_off)); \

The hypervisor side guest_handle_add_offset equivalents operate in
multiples of the type. If we can arrange that here too then I think that
would be good.

I think that means 
+                                 (_hcbuf_hnd2)->hbuf + ((sizeof(*_val)*(_off))); \

Then the caller becomes e.g.

set_xen_guest_handle_offset(sysctl.u.pcitopoinfo.devs, devs, processed);

which is much nicer IMHO.

Ian.