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[Qemu-devel] trace: timestamps, core IDs, and file creation

From: Hollis Blanchard
Subject: [Qemu-devel] trace: timestamps, core IDs, and file creation
Date: Wed, 13 Jan 2016 15:13:02 -0800
User-agent: Mozilla/5.0 (X11; Linux i686; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

Hi Stefan, I've been starting to use qemu tracing and found it quite useful. I have a couple comments about the trace events in general:

The event timestamps are host time (get_clock()). I'm correlating qemu events with other logs (using icount), so host time is unhelpful. Could we use cpu_get_clock() instead? (Trace events are used in other tools like qemu-io, where guest time doesn't exist, and there we could continue to use get_clock().)

When trying to understand multi-core guest behavior, it's pretty important to know which core is performing the traced action (e.g. MMIO). Would it make sense to automatically embed the core index, like the timestamp, or do you think it should be encoded in each individual tracepoint?

Right now, the simple tracepoint backend creates a trace file even when no events are active and no -trace options are present. Compounding the situation, omitting "-trace file=foo" results in the directory being littered with trace-NNNN files... could we create the file only when tracing is activated? And maybe with a more predictable name?

Hollis Blanchard
Mentor Graphics Emulation Division

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