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Date: Thu, 05 Sep 2013 03:26:25 +0200
From: Laszlo Ersek <lersek@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/2] add some virtio-scsi trace events
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: pbonzini@redhat.com, qemu-devel@nongnu.org

On 09/04/13 16:21, Stefan Hajnoczi wrote:
> On Thu, Aug 29, 2013 at 03:37:50PM +0200, Laszlo Ersek wrote:
>> +static void dump_cmd_req(const VirtIOSCSIReq *req, uint32_t cdb_size)
>> +{
>> +    const VirtIOSCSICmdReq *cr;
>> +    char *cdb_hex;
>> +
>> +    if (!trace_event_get_state(TRACE_VIRTIO_SCSI_DUMP_CMD_REQ)) {
>> +        return;
>> +    }
>> +    cr = req->req.cmd;
>> +    cdb_hex = qemu_hexstr(cr->cdb, cdb_size, NULL);
>> +    trace_virtio_scsi_dump_cmd_req((void *)req, cr->tag,
>> +                                   virtio_scsi_get_lun((uint8_t *)cr->lun),
>> +                                   cdb_hex);
>> +    g_free(cdb_hex);
> 
> There is a halfway solution to disable expensive trace events that works
> across all backends (SystemTap, stderr, etc):
> 
> if (!TRACE_VIRTIO_SCSI_DUMP_CMD_REQ_ENABLED) {
>     return;
> }
> 
> This is a compile-time constant which can be toggled with the "disable"
> keyword in the ./trace-events file:
> 
> disable my_expensive_event(const char *foo) "foo %s"
> 
> See the bottom of docs/tracing.txt for full documentation on the
> "disable" keyword.

Yes, I've read it :)

The compile-time constant is already used (just not in an if() but in a
surrounding #if, snipped from the context above), and the corresponding
tracepoints are already disabled in ./trace-events.

I do read documentation if it exists.

I guess I'll rework this sometime later.

Thanks,
Laszlo