* Rusty Russell ([email protected]) wrote:
> 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);
> ...
> }
>
There are other considerations that differs between the boot-time case
and the general "init" case: the write-protection flag must be
cleared-saved/restored when the kernel is running to patch read-only
text, but we don't want to modify cr0 at early boot on i386 because
paravirt is not executed yet (at boot time, pages are not
write-protected yet).
And I am not sure that it buys us anything to create an immediate_init()
when we can do exactly the same job with immediate_set. Yes, it might be
a bit slower, but we are not on a fast path.
>
> > > 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.
>
Yes, immediate values are used by the Linux Kernel Markers, which
instrument many code paths, including functions called from nmi and mce
contexts (including printk).
> > > 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.
>
Yes, doing arch specific calls without gcc support seems to be unlikely
to give us a neat portable solution.
Mathieu
> Rusty.
>
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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