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Re: [Qemu-devel] [PATCH] ehci: avoid string arguments in trace events

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH] ehci: avoid string arguments in trace events
Date: Mon, 5 Sep 2011 14:08:03 +0100

On Mon, Sep 5, 2011 at 9:38 AM, Gerd Hoffmann <address@hidden> wrote:
> On 09/03/11 17:22, Stefan Hajnoczi wrote:
>> String arguments are not supported by all trace backends.  This patch
>> replaces existing string arguments in hw/usb-ehci.c either with
>> individual trace events that remain human-friendly or by printing raw
>> addresses when there is no alternative or downside to that.
> Printing raw addresses *is* a downside.
>> States and usbsts bits remain human-friendly since it is hard to
>> remember all of them.  MMIO addresses are printed raw because they would
>> create many individual trace events and the addresses are usually easy
>> to remember when debugging.
> I find it hard to rememeber them.  There is a reason why the code to print
> the names for the mmio addresses is there in the first place. I don't want
> to loose that.
> Can't we just fix the backends instead?  Replacing debug fprintf with trace
> points isn't going to work if tracing can't handle strings.

I looked at the capabilities of the backends again and I think we
*can* allow strings.  The simple trace backend does not support them
but it's possible to add that.  I thought SystemTap doesn't either but
it turns out there is a userspace string copy function available.
Both stderr and ust are fine with strings.

Let's drop this patch.  I will update the tracing documentation.


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