On Fri, 2007-09-14 at 11:32 -0400, Mathieu Desnoyers wrote:
> * Rusty Russell ([email protected]) wrote:
> > Alternatively, if you called it "immediate_init" then the semantics
> > change slightly, but are more obvious (ie. only use this when the value
> > isn't being accessed yet). But it can't be __init then anyway.
> >
>
> I think your idea is good. immediate_init() could be used to update the
> immediate values at boot time _and_ at module load time, and we could
> use an architecture specific arch_immediate_update_init() to support it.
Right.
> As for "when" to use this, it should be used at boot time when
> interrupts are still disabled, still running in UP. It can also be used
> at module load time before any of the module code is executed, as long
> as the module code pages are writable (which they always are, for
> now..). Therefore, the flag seems inappropriate for module load
> arch_immediate_update_init. It cannot be put in __init section neither
> though if we use it like this.
I think from a user's POV it would be nice to have a 1:1 mapping with
normal initialization semantics (ie. it will work as long as you don't
access this value until initialized). And I think this would be the
case. eg:
int foo_func(void)
{
if (immediate_read(&some_immediate))
return 0;
...
}
int some_init(void)
{
immediate_init(some_immediate, 0);
register_foo(foo_func);
...
}
> > On an unrelated note, did you consider simply IPI-ing and doing the
> > substitution with all CPUs stopped? If you only updated the immediate
> > references to this particular var, it should be fast enough not to upset
> > the RT guys, even.
> >
>
> Yes, I thought about this, but since I use immediate values in the
> kernel markers, which can be put in exception handlers (including nmi,
> mce handler), which cannot be disabled without important side-effects, I
> don't think trying to stop the CPUs is a workable solution.
OK, but can you justify the use of immediates within the nmi or mce
handlers? They don't strike me as useful candidates for optimization.
> > Well, you can do that in asm without gcc support. It's a little nasty:
> > since gcc will know nothing about the function call, it can't have side
> > effects which are visible in this function, and you'll have to save and
> > restore *all* regs if you decide to do the function call. But it's
> > possible (a 5-byte nop gets changed to a call, the call does the pushes
> > and sets the args regs, calls the function, then pops everything and
> > rets).
>
> GCC support is required if we want to embed inline functions inside
> unlikely branches depending on immediate values (no function call
> there). It also permits passing local variables as arguments to the
> function call (stack setup), which would be tricky, instrumentation site
> specific and non portable if done in assembly.
Well if this is the slow path, you don't want inline anyway. But it
would be horribly, horribly arch-specific, yes.
Rusty.
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