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Date: Wed, 4 Sep 2013 16:21:05 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130904142105.GD12733@stefanha-thinkpad.redhat.com>
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	<1377783470-8981-2-git-send-email-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: Laszlo Ersek <lersek@redhat.com>
Cc: pbonzini@redhat.com, qemu-devel@nongnu.org

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.

Stefan