linux-kernel - Re: [PATCH] recordmcount: arm: Implement make

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Date:	Mon, 1 Feb 2016 11:44:32 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Rabin Vincent <rabin@....in>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH] recordmcount: arm: Implement make_nop

On 01/30, Rabin Vincent wrote:
> On Fri, Jan 29, 2016 at 05:18:06PM -0800, Stephen Boyd wrote:
> > diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> > index e167592793a7..0b16d14c54fb 100644
> > --- a/scripts/recordmcount.c
> > +++ b/scripts/recordmcount.c
> > @@ -206,6 +206,52 @@ static int make_nop_x86(void *map, size_t const offset)
> >  	return 0;
> >  }
> >  
> > +/*
> > + * Indicates if ARM is using __gnu_mcount_nc or mcount style and if
> > + * we should replace it with a pop or a nop respectively.
> > + */
> 
> For __gnu_mcount_nc, wouldn't it be better to replace both the push {lr}
> and the bl with nop instructions, instead of keeping a (useless) push +
> pop sequence?

Agreed. I was trying to do a 1-to-1 copy of the ftrace code on
ARM.  I was wondering the same thing in that code path while
doing this though. Can't we replace both instructions instead of
one instruction when we're patching in nops at runtime?

> 
> > +static int uses_altmcount;
> > +
> > +static unsigned char ideal_nop4_arm_arm[4] = { 0x00, 0x40, 0xbd, 0xe8 };
> > +static unsigned char ideal_nop4_arm_thumb[4] = { 0x5d, 0xf8, 0x04, 0xeb };
> > +static unsigned char ideal_nop4_arm_arm_be[4] = { 0xe8, 0xbd, 0x40, 0x00 };
> > +static unsigned char ideal_nop4_arm_thumb_be[4] = { 0xf8, 0x5d, 0xeb, 0x04 };
> > +static unsigned char ideal_nop4_arm_old[4] = { 0x00, 0x00, 0xa0, 0xe1 };
> > +static unsigned char ideal_nop4_arm_old_be[4] = { 0xe1, 0xa0, 0x00, 0x00 };
> > +
> > +static unsigned char bl_gnu_mcount_nc_arm[4] = { 0xfe, 0xff, 0xff, 0xeb };
> > +static unsigned char bl_gnu_mcount_nc_thumb[4] = { 0xff, 0xf7, 0xfe, 0xff };
> > +static unsigned char bl_gnu_mcount_nc_arm_be[4] = { 0xeb, 0xff, 0xff, 0xfe };
> > +static unsigned char bl_gnu_mcount_nc_thumb_be[4] = { 0xf7, 0xff, 0xff, 0xfe };
> 
> Comments showing what assembly instructions all these correspond to
> would be helpful.

Sure.

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