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Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel

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On Wed, 2016-02-17 at 12:30 +0100, Torsten Duwe wrote:
> On Wed, Feb 17, 2016 at 09:55:40PM +1100, Michael Ellerman wrote:
> > 
> > On a kernel built with the 2 instruction version this will fault when the
> > function we're looking at is located at the beginning of a page. Because
> > instruction[-3] goes off the front of the mapping.
> > 
> > We can probably fix that. But it's still a bit dicey.
> 
> Not necessarily. Now that it's a separate function, it can be nested a bit deeper,
> so we don't take chances on compiler optimisation:
> 
> if (instruction[-2] == PPC_INST_STD_LR) /* where should R0 come from? there must be... */
>   {
>     if (instruction[-3] == PPC_INST_MFLR)
>       return 1;
>   }
> else if (instruction[-2] == PPC_INST_MFLR)
>     return 1;
> return 0;

Yeah true that should work in practice.

It's still trivial to construct a module that will oops the loader, but I guess
that's always been true.

> > I'm wondering if we want to just say we only support the 2 instruction version.
> > Currently that means GCC 6 only, or a distro compiler with the backport of
> > e95d0248dace. But we could also ask GCC to backport it to 4.9 and 5.
> 
> IMHO that's a too weak reason for a too strong limitation. OTOH getting everyone
> to use the 2 insn version sounds appealing...

Fair enough. I'm just trying to manage the complexity explosion.

I'd certainly advocate that you backport it to your toolchain.

> Is e95d0248dace self-sufficient or does it depend on other improvements?

AFAIK it's self sufficient, it just deletes a single line. I'll ask the GCC
guys tomorrow if they can backport it if you don't beat me to it :)

cheers

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[PATCH v8 0/8] ftrace with regs + live patching for ppc64 LE (ABI v2) Torsten Duwe <duwe@lst.de> - 2016-02-10 18:50 +0100
  [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe <duwe@lst.de> - 2016-02-10 18:50 +0100
    Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Balbir Singh <bsingharora@gmail.com> - 2016-02-11 08:50 +0100
      Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> - 2016-02-11 09:50 +0100
        Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Balbir Singh <bsingharora@gmail.com> - 2016-02-11 10:40 +0100
          Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Murali Sampath <MSampath@alphion.com> - 2016-02-11 14:10 +0100
          Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Murali Sampath <MSampath@alphion.com> - 2016-02-11 14:10 +0100
      Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe <duwe@lst.de> - 2016-02-11 09:50 +0100
        Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Balbir Singh <bsingharora@gmail.com> - 2016-02-11 10:40 +0100
        Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Michael Ellerman <mpe@ellerman.id.au> - 2016-02-15 11:30 +0100
          Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Jiri Kosina <jikos@kernel.org> - 2016-02-15 14:00 +0100
          Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe <duwe@lst.de> - 2016-02-15 15:10 +0100
            Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe <duwe@lst.de> - 2016-02-15 23:30 +0100
              Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Balbir Singh <bsingharora@gmail.com> - 2016-02-16 06:00 +0100
            Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Michael Ellerman <mpe@ellerman.id.au> - 2016-02-16 11:20 +0100
              Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe <duwe@lst.de> - 2016-02-16 11:40 +0100
  [PATCH v8 3/8] ppc use ftrace_modify_all_code default Torsten Duwe <duwe@lst.de> - 2016-02-10 18:50 +0100
  [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel Torsten Duwe <duwe@lst.de> - 2016-02-10 18:50 +0100
    Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel Michael Ellerman <mpe@ellerman.id.au> - 2016-02-17 12:00 +0100
      Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel Michael Ellerman <mpe@ellerman.id.au> - 2016-02-17 12:40 +0100
      Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel Torsten Duwe <duwe@lst.de> - 2016-02-17 12:40 +0100
  [PATCH v8 6/8] Implement kernel live patching for ppc64le (ABIv2) Torsten Duwe <duwe@lst.de> - 2016-02-10 18:50 +0100
    Re: [PATCH v8 6/8] Implement kernel live patching for ppc64le  (ABIv2) Miroslav Benes <mbenes@suse.cz> - 2016-02-11 10:10 +0100
  Re: [PATCH v8 0/8] ftrace with regs + live patching for ppc64 LE  (ABI v2) Balbir Singh <bsingharora@gmail.com> - 2016-02-11 07:20 +0100
    Re: [PATCH v8 0/8] ftrace with regs + live patching for ppc64 LE  (ABI v2) Torsten Duwe <duwe@lst.de> - 2016-02-11 09:40 +0100

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