Re: [Qemu-devel] [PATCH] net/vmxnet3.c: fix a build error when enabling debug output

[Eric Blake]

On 12/02/2015 10:08 PM, Miao Yan wrote:
> Macro MAC_FMT and MAC_ARG are not defined, but used in vmxnet3_net_init().
> This will cause build error when debug level is raised in
> vmxnet3_debug.h (enable all VMXNET3_DEBUG_xxx).
> 
> Use VMXNET_MF and VXMNET_MA instead.
> 
> Signed-off-by: Miao Yan <address@hidden>
> ---
>  hw/net/vmxnet3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/net/vmxnet3.c b/hw/net/vmxnet3.c
> index 5e3a233..ea3d9b7 100644
> --- a/hw/net/vmxnet3.c
> +++ b/hw/net/vmxnet3.c
> @@ -2044,7 +2044,7 @@ static void vmxnet3_net_init(VMXNET3State *s)
>  
>      s->link_status_and_speed = VMXNET3_LINK_SPEED | VMXNET3_LINK_STATUS_UP;
>  
> -    VMW_CFPRN("Permanent MAC: " MAC_FMT, MAC_ARG(s->perm_mac.a));
> +    VMW_CFPRN("Permanent MAC: " VMXNET_MF, VMXNET_MA(s->perm_mac.a));

This is a classic example of why dead code debug statements are evil.

You should consider also providing a patch to hw/net/vmxnet_debug.h to
fix ALL of the broken debug macros in that file to instead use a sane
pattern, so that the format string is ALWAYS compiled and just optimized
out when debugging is disabled.

Here's a conversion of one of the macros for an example of what to do:

Instead of:

> #ifdef VMXNET_DEBUG_CONFIG
> #define VMW_CFPRN(fmt, ...)                                                 \
>     do {                                                                    \
>         printf("[%s][CF][%s]: " fmt "\n", VMXNET_DEVICE_NAME, __func__,     \
>             ## __VA_ARGS__);                                                \
>     } while (0)
> #else
> #define VMW_CFPRN(fmt, ...) do {} while (0)
> #endif

you should do:

#ifdef VMXNET_DEBUG_CONFIG
# define VMXNET_DEBUG_CONFIG_FLAG 1
#else
# define VMXNET_DEBUG_CONFIG_FLAG 0
#endif
#define VMW_CFPRN(fmt, ...)                                       \
    do {                                                          \
        if (VMXNET_DEBUG_CONFIG_FLAG) {                           \
            printf("[%s][CF][%s]: " fmt "\n", VMXNET_DEVICE_NAME, \
                   __func__, ## __VA_ARGS__);                     \
        }                                                         \
    } while (0);

With that pattern, VMW_CFPRN() will now always check that its arguments
can compile, even though it has no impact to the code size when
VMXNET_DEBUG_CONFIG is not defined.  Note that once you repair all of
the broken macros (I count 8 in that file), you may have some fallout of
other broken dead code that needs to be fixed.

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

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