Re: [PATCH 2/3] c/r: Add CR_COPY() macro (v2)

[Nathan Lynch <ntl-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>]

On Wed, 25 Feb 2009 13:12:08 -0500
Dan Smith <danms-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> wrote:

> As suggested by Dave, this provides us a way to make the copy-in and
> copy-out processes symmetric.  I also added CR_COPY_BIT() to use with
> the s390 bitfields, since we can't memcpy() those.
> 
> Changelog:
>     Feb 25:
>             . Changed WARN_ON() to BUILD_BUG_ON()
> 
> Signed-off-by: Dan Smith <danms-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
> ---
>  include/linux/checkpoint.h |   20 ++++++++++++++++++++
>  1 files changed, 20 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
> index 217cf6e..3add90e 100644
> --- a/include/linux/checkpoint.h
> +++ b/include/linux/checkpoint.h
> @@ -149,4 +149,24 @@ static inline void process_deny_checkpointing(struct task_struct *task) {}
>  
>  #endif
>  
> +#define CR_CPT 1
> +#define CR_RST 2
> +
> +#define CR_COPY(op, a, b)				\
> +	do {						\
> +		BUILD_BUG_ON(sizeof(a) != sizeof(b));	\
> +		if (op == CR_CPT)			\
> +			memcpy(&a, &b, sizeof(a));	\
> +		else					\
> +			memcpy(&b, &a, sizeof(a));	\
> +	} while (0);
> +
> +#define CR_COPY_BIT(op, a, b)				\
> +	do {						\
> +		if (op == CR_CPT)			\
> +			a = b;				\
> +		else					\
> +			b = a;				\
> +	} while (0);
> +
>  #endif /* _CHECKPOINT_CKPT_H_ */

No typechecking.  Multiple expansion of arguments (probably harmless
for the current use case, but still).  Generates a memcpy where,
depending on the arguments, simple assignment would be sufficient and
preferred.  Not useful for targets where fields in the checkpoint image
are larger than the register state they reflect (32-bit variants of x86
and powerpc).

Anyway, checkpoint and restart should not be "symmetric" -- the restart
code has to validate certain values, such as privileged registers, in
the image before committing them.

And while these macros may reduce initial development effort, the
reading effort incurred is non-trivial.  It doesn't seem worth it.