Re: [Qemu-devel] [PATCH for-2.10 0/2] trace: fix simpletrace.stp flight recorder mode

[Stefan Hajnoczi]

On Mon, Aug 14, 2017 at 05:39:08PM +0100, Daniel P. Berrange wrote:
> On Mon, Aug 14, 2017 at 05:21:40PM +0100, Stefan Hajnoczi wrote:
> > The SystemTap flight recorder mode no longer works with simpletrace.stp 
> > because
> > the event ID mapping records are emitted the first time an event fires.
> > Chances are, the event ID mapping record will not be in ring buffer when the
> > users wants to print the trace.
> > 
> > This series solves the issue by using the trace-events-all global event
> > ordering for event IDs for simpletrace.stp.
> 
> I see the way you've written it means it'll still honour the event
> id mappings in the header, if present, but I wonder what's the point
> of keeping that anymore ? If we need to support this ring-buffer mode,
> there's no way we can take advantage of having the ID mappings dynamically
> defined - we must always guarantee a stable mapping.
> 
> IOW, should we just revert
> 
>   commit 7f1b588f20d027730676e627713ae3bbf6baab04
>   Author: Daniel P. Berrange <address@hidden>
>   Date:   Tue Oct 4 14:35:50 2016 +0100
> 
>     trace: emit name <-> ID mapping in simpletrace header
>     
> entirely ?

We cannot revert the commit because QEMU dynamically assigns event IDs
on startup.  Event IDs are not known ahead of time when the 'simple'
tracer is used.

Event IDs are only static when simpletrace.stp is used with the 'dtrace'
tracer since simpletrace.stp has global knowledge of all trace events
ahead of time.

Stefan

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