Re: user instruction tracing patch?

[Roland McGrath <roland at redhat dot com>]

> Since utrace will provide the pt_regs structure, the probe handler
> bodies will be able to call e.g. backtrace(), probefunc(), and really
> should have some structured access to the registers (a new tapset
> function like register:long ("name") ?)

In general to access all registers you can't always use pt_regs directly;
the details vary by machine.  You can get many of them on the common
machines (x86, ppc).  A simple embedded-C tapset approach for each machine
is certainly an easy place to start.  For full generality, you'll need to
use the formal utrace_regset interfaces instead.  The elfutils libraries
have full information on mapping register names and DWARF numbers to the
formats these interfaces expose, though they could use more sugar.  There
is a variety of ways you could use that info in the translator to expose
register access to the script language.

>   probe process(PID).itrace if (condition) { }

So this is special syntax constrained to just testing a script variable?
Or it's any complex expression reducible to only reading global script
variables?  Or what exactly?  

I like this syntax when it's just a general thing for skipping out probe
hits.  (In fact, you can call it awk and say it's really just a
one-statement probe body since {...} is just a statement.)  

But when its real use is to cause acts that change the value of evaluating
the condition to hook into immediately changing the state of low-level
tracing features that produce the probe hits, then I want to know exactly
what the language semantics guarantees about the timely control of that state.

>   probe process(PID).function("NAME") { condition = 1 }
>   probe process(PID).function("NAME").return { condition = 0 }

For likely answers to my first question, doesn't seem much different from
an explicit enable/disable command, just more obscure when reading the script.
If the variable is not otherwise used, wouldn't that compile to the same as:

    probe process(PID).function("NAME") { enable process(PID).itrace; }
    probe process(PID).function("NAME").return { disable process(PID).itrace; }

or whatever syntax explicit enable/disable has?


Thanks,
Roland