RE: [RFC PATCH v4 12/17] powerpc64/ftrace: Move ftrace sequence out of line

[David Laight <David.Laight@ACULAB.COM>]

From: Nicholas Piggin
> Sent: 15 July 2024 09:25
> 
> On Sun Jul 14, 2024 at 6:27 PM AEST, Naveen N Rao wrote:
> > Function profile sequence on powerpc includes two instructions at the
> > beginning of each function:
> > 	mflr	r0
> > 	bl	ftrace_caller
> >
> > The call to ftrace_caller() gets nop'ed out during kernel boot and is
> > patched in when ftrace is enabled.
> >
> > Given the sequence, we cannot return from ftrace_caller with 'blr' as we
> > need to keep LR and r0 intact. This results in link stack (return
> > address predictor) imbalance when ftrace is enabled. To address that, we
> > would like to use a three instruction sequence:
> > 	mflr	r0
> > 	bl	ftrace_caller
> > 	mtlr	r0
> >
> > Further more, to support DYNAMIC_FTRACE_WITH_CALL_OPS, we need to
> > reserve two instruction slots before the function. This results in a
> > total of five instruction slots to be reserved for ftrace use on each
> > function that is traced.
> >
> > Move the function profile sequence out-of-line to minimize its impact.
> > To do this, we reserve a single nop at function entry using
> > -fpatchable-function-entry=1 and add a pass on vmlinux.o to determine
> > the total number of functions that can be traced. This is then used to
> > generate a .S file reserving the appropriate amount of space for use as
> > ftrace stubs, which is built and linked into vmlinux.
> 
> These are all going into .tramp.ftrace.text AFAIKS? Should that be
> moved after some of the other text in the linker script then if it
> could get quite large? sched and lock and other things should be
> closer to the rest of text and hot code.

Can't you allocate the space for the 'function profile sequence'
at run-time when (if) ftrace is enabled?
When ftrace gets disabled it is likely possible to save the trampoline
number in the nop - so the same memory can be used next time.

	David

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