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xen-devel
Re: [Xen-devel] unconditionally enable the trace buffer
> >* ability to turn on/off via hypercall
>
> Not currently implemented, but would not be difficult to add.
Just as an aside I'm not sure this matters: From what Rob's told me, having
the (inline) trace() calls in there produces the same overhead whether
tracing is active or not. I guess it makes sense; once you've incurred the
overhead of having the function there and evaluating the "is tracing on"
conditional, you might as well have stored a few values also ;-)
> >* ability to set the per CPU tracebuffer size when turning it on
>
> Partially; You can enable the trace buffer on the xen (boot) command
> line, and you can specify the trace buffer size there. You cannot change
> the size dynamically.
Aside #2: You can disable at boot time by setting size=0.
> >* ability for the user-space reader to explicitly block (select on fd)
> >on an eventchn notification that the buffer is e.g. half full. (reader
> >should write out all the pages that are full of trace events)
>
> Not done.
We can just block on /dev/evtchn, I guess. Shouldn't be too hard; we just
need to define a new "trace" virq.
> >* user space reader should log when it misses blocks of events
> >(overwrite last trace message in buffer with a special 'missed X events'
> >message)
>
> Not done, but my XenMon patch includes a change to the trace buffer code
> to help with this. I've added a "sequence number" to each trace record
> which can be used to detect when blocks of events have been missed.
Yep, sounds good to me.
Cheers,
Mark0
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