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From: Jordan Niethe <jniethe5@gmail.com>
Date: Wed, 5 May 2021 15:22:17 +1000
Message-ID: <CACzsE9qSFNqYD7kf5G_6s8evDdjBJbsczBHHiZUvdTPWEvCz7g@mail.gmail.com>
Subject: Re: [PATCH v11 2/9] powerpc/lib/code-patching: Set up Strict RWX
 patching earlier
To: Christophe Leroy <christophe.leroy@csgroup.eu>
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On Thu, Apr 29, 2021 at 2:53 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 29/04/2021 =C3=A0 05:15, Jordan Niethe a =C3=A9crit :
> > setup_text_poke_area() is a late init call so it runs before
> > mark_rodata_ro() and after the init calls. This lets all the init code
> > patching simply write to their locations. In the future, kprobes is
> > going to allocate its instruction pages RO which means they will need
> > setup_text__poke_area() to have been already called for their code
> > patching. However, init_kprobes() (which allocates and patches some
> > instruction pages) is an early init call so it happens before
> > setup_text__poke_area().
> >
> > start_kernel() calls poking_init() before any of the init calls. On
> > powerpc, poking_init() is currently a nop. setup_text_poke_area() relie=
s
> > on kernel virtual memory, cpu hotplug and per_cpu_areas being setup.
> > setup_per_cpu_areas(), boot_cpu_hotplug_init() and mm_init() are called
> > before poking_init().
> >
> > Turn setup_text_poke_area() into poking_init().
>
> I can't remember, maybe I already asked the question:
> Have you done some performance measurement or at least some performance a=
nalysis ?
No I don't think you have asked and it is a good question.

Here are some results on a Power9 (T2P9D01 REV 1.01) running powernv_defcon=
fig
Timestamp at "Run /init as init process"
Before: ~1.059326
After: ~1.273105

Turning on more testing the difference is greater:
For example, turning on CONFIG_FTRACE_STARTUP_TEST
Timestamp at "Run /init as init process"
Before: ~7.176759
After: ~15.967576

Running with initcall_debug:
Before: initcall init_trace_selftests+0x0/0x1b4 returned 0 after 2880859 us=
ecs
After: initcall init_trace_selftests+0x0/0x1b4 returned 0 after 10048828 us=
ecs

So it does slow it down.
But it also might be a good thing for testing that these tests using
code patching now will use the same code path for patching that would
be used on a fully booted system.

>
> Christophe
>
> >
> > Reviewed-by: Russell Currey <ruscur@russell.cc>
> > Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
> > ---
> > v9: New to series
> > ---
> >   arch/powerpc/lib/code-patching.c | 12 ++++--------
> >   1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-p=
atching.c
> > index 870b30d9be2f..15296207e1ba 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -70,14 +70,11 @@ static int text_area_cpu_down(unsigned int cpu)
> >   }
> >
> >   /*
> > - * Run as a late init call. This allows all the boot time patching to =
be done
> > - * simply by patching the code, and then we're called here prior to
> > - * mark_rodata_ro(), which happens after all init calls are run. Altho=
ugh
> > - * BUG_ON() is rude, in this case it should only happen if ENOMEM, and=
 we judge
> > - * it as being preferable to a kernel that will crash later when someo=
ne tries
> > - * to use patch_instruction().
> > + * Although BUG_ON() is rude, in this case it should only happen if EN=
OMEM, and
> > + * we judge it as being preferable to a kernel that will crash later w=
hen
> > + * someone tries to use patch_instruction().
> >    */
> > -static int __init setup_text_poke_area(void)
> > +int __init poking_init(void)
> >   {
> >       BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> >               "powerpc/text_poke:online", text_area_cpu_up,
> > @@ -85,7 +82,6 @@ static int __init setup_text_poke_area(void)
> >
> >       return 0;
> >   }
> > -late_initcall(setup_text_poke_area);
> >
> >   /*
> >    * This can be called for kernel text or a module.
> >