Re: [PATCH] add -fprolog-pad=N option to c-family

[Torsten Duwe <duwe at suse dot de>]

On Thu, Apr 28, 2016 at 11:58:25AM +0100, Szabolcs Nagy wrote:
> On 28/04/16 09:47, Maxim Kuvyrkov wrote:
> >> On Apr 27, 2016, at 7:26 PM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> >>
> >> with -mfentry, by default the user only has to
> >> implement the fentry call (linux wants nops there, but
> >> e.g. glibc could use -pg -mfentry for profiling on
> >> aarch64 and the target specific details are easier to
> >> document for an -m option than for something general).
> > 
> > I don't understand your point here, could you elaborate, please?
> > 
> 
> if we only provide -mfentry then
> 
> - the kernel can use it (they have tools to nop patch the binary),
> 
> - others who don't want to fiddle with nops, just have the call,
> can also use it (e.g. user-space profiling cannot really use
> something that needs binary patching in case the user prefers
> -pg -mfentry over the current -pg behaviour).

Any examples of users not satisfied with the current -pg ;-) ?

> - it's target specific, so the magic abi of the fentry call can
> be documented by the target according to the specific instruction

There's a downside to this: you will have to reimplement it in gcc

  * for every architecture
  * for every ABI variant

while the generic approach is -- well -- somewhat generic :-]

> sequence that is used. (with nop-padding there are psabi and
> compiler optimization interactions that may be hard to document
> in a generic way and letting the user figure it out may cause
> problems later in compiler development.. but i'm just speculating
> based on the powerpc toc handling and ipa-ra findings.)

ipa-ra is from hell ;) At least from a function-patcher's standpoint.
You may argue that OTOH function binary patching is from hell :)

> >> the nop-padding is more general, but the size and
> >> layout of nops and the call abi will be target
> >> specific and the user will most likely need to modify
> >> the binary (to get the right sequence) which needs
> >> additional tooling.  i don't know who might use it
> >> other than linux (which already has tools to deal with
> >> -mfentry).

On exactly 1 (one!) architecture. s390x uses NOP padding, hint, hint...

> i'm trying to find where this happens in the kernel, but
> i only see scripts/recordmcount.{c,pl} which are based on
> nop patching the fentry/mcount call sites.
> 
> without such call sites the tools have to be implemented
> differently and the way the kernel records the call site
> positions might not match the prolog-pad recording.

AFAICS Maxim has provided a nice mechanism to find the NOP pads.

Let's see how far we can get, then discuss this further,
I suggest.

	Torsten