[lttng-dev] Making 32-bit user-space events on a 64-bit Linux system

[Thibault, Daniel]

-----Message d'origine-----
De : Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com] 
Envoyé : 7 octobre 2013 08:57

----- Original Message -----
> From: "Alexandre Montplaisir" <alexmonthy at voxpopuli.im>
> Sent: Thursday, October 3, 2013 11:54:15 AM
> 
> >    On a different note, why does LTTng have 32- and 64-bit consumers
> >    anyway?  The only possible difference between a 32-bit tracer and 
> > a 64-bit tracer is that the latter could generate event record 
> > fields that are 64-bit longs and floats, while the former cannot 
> > (but will never have to, since the application it is tracing won't 
> > have such beasts anyway).
>
> No, this is not it at all. 32-bit apps can generate 64-bit integers too, and double-precision floats.

   I realised that soon after having written the above, when I looked up the C spec.  Clearly, the event records are precisely the same in 32- and 64-bit modes.  But if the event buffers don't have intrinsic bitness, something else must be responsible for the requirement for 32-bit consumers.  Does mmap have a bitness barrier?  Or do the event buffers have intrinsic bitness in the sense that the buffer management fields change?

> The difference between 32 and 64-bit consumers mainly comes for atomic operations: on 64-bit,
> consumer and application can interact through atomic operations on 64-bit integers, which allow
> detection of free-running counter overflow much larger than with 32-bit counter.
>
> Mathieu

   If I understand you correctly, the 64-bit atomic operations needed are not available in the 32-bit assembly code.  Makes sense.  One also sees that the dynamic loader has 32- and 64-bit operating modes, due mostly to different ELF formats.  But is there anything in the processor architecture that makes it impossible (or undesirable because of, say, some sort of context-switching) for a 64-bit processor running 32-bit code to insert 64-bit instructions in the code flow?  Alternately, how hard is it for 32-bit code to call a 64-bit library?  (In some ways, this is similar to inserting an external call or a piece of assembly code in high-level language code such as Object Pascal)  Taking yet another step back, what is it about the event buffer lying in a 32-bit process's space that makes it undesirable/difficult/impossible for a 64-bit consumer to access it?
   Obviously, when running on a 32-bit processor LTTng has no choice but to be 32-bit wide throughout, and therefore it cannot use the desirable 64-bit integer atomic comparisons.  Running on a 64-bit processor should be a wholly different kettle of fish, however.

Daniel U. Thibault
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