Re: [Xen-devel] [PATCH v4 1/9] libxc: add DECLARE_HYPERCALL_BUFFER_SHADOW()

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-04-30 at 19:36 +0100, Andrew Cooper wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> DECLARE_HYPERCALL_BUFFER_SHADOW() is like DECLARE_HYPERCALL_BUFFER()
> except it is backed by an already allocated hypercall buffer.

I suppose enhancing DECLARE_HYPERCALL_BUFFER_ARGUMENT to have this
property has issues with unused variables?

HYPERCALL_BUFFER_AS_PTR() would have been an alternative implementation
(similar to AS_ARG), I suppose there is no particular reason to prefer
one over the other?

> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  tools/libxc/xenctrl.h |   18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/tools/libxc/xenctrl.h b/tools/libxc/xenctrl.h
> index 02129f7..15d2f4f 100644
> --- a/tools/libxc/xenctrl.h
> +++ b/tools/libxc/xenctrl.h
> @@ -266,6 +266,24 @@ typedef struct xc_hypercall_buffer xc_hypercall_buffer_t;
>      }
>  
>  /*
> + * Like DECLARE_HYPERCALL_BUFFER() but using an already allocated
> + * hypercall buffer, _hbuf.
> + *
> + * Useful when a hypercall buffer is passed to a function and access
> + * via the user pointer is required.
> + *
> + * See DECLARE_HYPERCALL_BUFFER_ARGUMENT() if the user pointer is not
> + * required.
> + */
> +#define DECLARE_HYPERCALL_BUFFER_SHADOW(_type, _name, _hbuf)   \
> +    _type *_name = _hbuf->hbuf;                                \
> +    xc_hypercall_buffer_t XC__HYPERCALL_BUFFER_NAME(_name) = { \
> +        .hbuf = (void *)-1,                                    \
> +        .param_shadow = _hbuf,                                 \
> +        HYPERCALL_BUFFER_INIT_NO_BOUNCE                        \
> +    }
> +
> +/*
>   * Declare the necessary data structure to allow a hypercall buffer
>   * passed as an argument to a function to be used in the normal way.
>   */



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