Re: [Qemu-devel] [RFC PATCH] cputlb: modernise the debug support

[Eric Blake <eblake@redhat.com>]

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On 06/05/2015 09:55 AM, Alex Bennée wrote:
> To avoid cluttering the code with #ifdef legs we wrap up the print
> statements into a tlb_debug() macro. As access to the virtual TLB can
> get quite heavy defining DEBUG_TLB_LOG will ensure all the logs go to
> the qemu_log target of CPU_LOG_MMU instead of stderr.
> 
> I've also removed DEBUG_TLB_CHECK which wasn't used.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  cputlb.c | 47 ++++++++++++++++++++++++++++-------------------
>  1 file changed, 28 insertions(+), 19 deletions(-)
> 
> diff --git a/cputlb.c b/cputlb.c
> index 7606548..ddb7b59 100644
> --- a/cputlb.c
> +++ b/cputlb.c
> @@ -30,8 +30,23 @@
>  #include "exec/ram_addr.h"
>  #include "tcg/tcg.h"
>  
> -//#define DEBUG_TLB
> -//#define DEBUG_TLB_CHECK
> +/* DEBUG defines, enable DEBUG_TLB_LOG to log to the CPU_LOG_MMU target */
> +/* #define DEBUG_TLB */
> +/* #define DEBUG_TLB_LOG */
> +
> +#ifdef DEBUG_TLB
> +#ifdef DEBUG_TLB_LOG
> +#define tlb_debug(fmt, ...) do {                                        \
> +        qemu_log_mask(CPU_LOG_MMU, "%s: " fmt, __func__, ## __VA_ARGS__); \
> +    } while (0)
> +#else
> +#define tlb_debug(fmt, ...) do {                               \
> +        fprintf(stderr, "%s: " fmt, __func__, ## __VA_ARGS__); \
> +    } while (0)
> +#endif
> +#else
> +#define tlb_debug(fmt, ...) do { } while (0)

This is prone to bitrot.

Better would be:

#ifdef DEBUG_TLB
# define DEBUG_TLB_GATE 1
# ifdef DEBUG_TLB_LOG
#  define DEBUG_TLB_LOG_GATE 1
# else
#  define DEBUG_TLB_LOG_GATE 0
# endif
#else
# define DEBUG_TLB_GATE 0
# define DEBUG_TLB_LOG_GATE 0
#endif

#define tlb_debug(fmt, ...) do { \
    if (DEBUG_TLB_LOG_GATE) { \
        qemu_log_mask(CPU_LOG_MMU, "%s: " fmt, __func__, \
                      ## __VA_ARGS__); \
    } else if (DEBUG_TLB_GATE) { \
        fprintf(stderr, "%s: " fmt, __func__, ## __VA_ARGS__); \
    } \
} while (0)

because then tlb_debug() will automatically guarantee compiler
compliance to correct fmt vs. argument, while still benefitting from no
increase in code size (all compilers can properly optimize if(0) blocks).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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