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Date: Thu, 13 Nov 2014 19:49:40 +0800
From: Gonglei <arei.gonglei@huawei.com>
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Subject: Re: [Qemu-devel] [PATCH] virtio-bus: avoid breaking build when open
 DEBUG switch
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: "qemu-trivial@nongnu.org" <qemu-trivial@nongnu.org>, "mst@redhat.com" <mst@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>

On 2014/11/13 19:27, Stefan Hajnoczi wrote:

> On Thu, Nov 13, 2014 at 10:39:32AM +0800, arei.gonglei@huawei.com wrote:
>> From: Gonglei <arei.gonglei@huawei.com>
>>
>> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>> ---
>>  hw/virtio/virtio-bus.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
>> index eb77019..dfd2d8c 100644
>> --- a/hw/virtio/virtio-bus.c
>> +++ b/hw/virtio/virtio-bus.c
>> @@ -58,7 +58,7 @@ void virtio_bus_reset(VirtioBusState *bus)
>>  {
>>      VirtIODevice *vdev = virtio_bus_get_device(bus);
>>  
>> -    DPRINTF("%s: reset device.\n", qbus->name);
>> +    DPRINTF("%s: reset device.\n", BUS(bus)->name);
> 
> In general the problem with existing DPRINTF() macros is that they use
> #ifdef.  This leads to bitrot, such as this instance.
> 

Yes.

> A better approach is:
> 
> #define DEBUG_FOO 0
> #define DPRINTF(...) \
> do { \
>     if (DEBUG_FOO) { \
>         fprintf(stderr, ...); \
>     } \
> } while (0)
> 
> Now the compiler always checks the DPRINTF() code.
> 
> Trace events can be a good solution too.  If you like stderr output,
> build QEMU with ./configure --enable-trace-backend=stderr and all
> trace_*() calls are turned into fprintf(stderr).
> 
> The cool thing about trace events is that they are available in
> production too.
> 

Hum, trace events is great! :)

Best regards,
-Gonglei