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From: =?utf-8?Q?Llu=C3=ADs_Vilanova?= <vilanova@ac.upc.edu>
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Date: Thu, 18 Aug 2016 12:32:14 +0200
In-Reply-To: <20160818095945.GB4850@stefanha-x1.localdomain> (Stefan
	Hajnoczi's message of "Thu, 18 Aug 2016 10:59:45 +0100")
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Subject: Re: [Qemu-devel] [PATCH 2/6] hypertrace: Add tracing event
 "guest_hypertrace"
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

Stefan Hajnoczi writes:

> On Fri, Aug 05, 2016 at 06:59:34PM +0200, Llu=C3=ADs Vilanova wrote:
>> +# hypertrace
>> +hyperargs=3D$hypertrace
>> +if test $hypertrace =3D "disabled"; then
>> +    hyperargs=3D0
>> +fi
>> +echo "CONFIG_HYPERTRACE_ARGS=3D$hyperargs" >> $config_host_mak
>> +hypertrace_events=3Dhypertrace/trace-events
>> +mkdir -p $(dirname $hypertrace_events)
>> +echo "# See docs/trace-events.txt for syntax documentation." >$hypertra=
ce_events
>> +echo -n 'vcpu guest_hypertrace(' >>$hypertrace_events
>> +for i in `seq $hypertrace`; do
>> +    if test $i !=3D 1; then
>> +        echo -n ", " >>$hypertrace_events
>> +    fi
>> +    echo -n "uint64_t arg$i" >>$hypertrace_events
>> +done
>> +echo -n ') ' >>$hypertrace_events
>> +for i in `seq $hypertrace`; do
>> +    echo -n "\" arg$i=3D0x%016\"PRIx64" >>$hypertrace_events
>> +done
>> +echo >>$hypertrace_events

> This reminds me of the first versions of "simpletrace" where the number
> of arguments was fixed and argument size was fixed.

> This meant strings cannot be traced, number of arguments is limited, and
> you pay an space overhead for unused arguments.

> Later on the format was changed to header (including .length field) and
> binary data payload.  This reduced the space overhead, elminated the
> argument count limit, and allowed strings to be traced.

> I think these are desirable qualities for any tracing mechanism and
> would reconsider a fixed number of uint64_t arguments.

The number of arguments is fixed at *compilation time*. What is not support=
ed
out of the box is multiplexing multiple guest events over the hypertrace
event. That is, you need to do it yourself when post-processing the traces,=
 but
I don't think that's too bad.


Cheers,
  Lluis