Re: [Xen-devel] unconditionally enable the trace buffer

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> >* 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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