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Date: Wed, 26 Feb 2020 17:06:43 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v3 09/14] powerpc/xmon: Add initial support for prefixed
 instructions
To: Jordan Niethe <jniethe5@gmail.com>, linuxppc-dev@lists.ozlabs.org
References: <20200226040716.32395-1-jniethe5@gmail.com>
 <20200226040716.32395-10-jniethe5@gmail.com>
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Jordan Niethe's on February 26, 2020 2:07 pm:
> A prefixed instruction is composed of a word prefix and a word suffix.
> It does not make sense to be able to have a breakpoint on the suffix of
> a prefixed instruction, so make this impossible.
>=20
> When leaving xmon_core() we check to see if we are currently at a
> breakpoint. If this is the case, the breakpoint needs to be proceeded
> from. Initially emulate_step() is tried, but if this fails then we need
> to execute the saved instruction out of line. The NIP is set to the
> address of bpt::instr[] for the current breakpoint.  bpt::instr[]
> contains the instruction replaced by the breakpoint, followed by a trap
> instruction.  After bpt::instr[0] is executed and we hit the trap we
> enter back into xmon_bpt(). We know that if we got here and the offset
> indicates we are at bpt::instr[1] then we have just executed out of line
> so we can put the NIP back to the instruction after the breakpoint
> location and continue on.
>=20
> Adding prefixed instructions complicates this as the bpt::instr[1] needs
> to be used to hold the suffix. To deal with this make bpt::instr[] big
> enough for three word instructions.  bpt::instr[2] contains the trap,
> and in the case of word instructions pad bpt::instr[1] with a noop.
>=20
> No support for disassembling prefixed instructions.
>=20
> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
> ---
> v2: Rename sufx to suffix
> v3: - Just directly use PPC_INST_NOP
>     - Typo: plac -> place
>     - Rename read_inst() to mread_inst(). Do not have it call mread().
> ---
>  arch/powerpc/xmon/xmon.c | 90 ++++++++++++++++++++++++++++++++++------
>  1 file changed, 78 insertions(+), 12 deletions(-)
>=20
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a673cf55641c..a73a35aa4a75 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -97,7 +97,8 @@ static long *xmon_fault_jmp[NR_CPUS];
>  /* Breakpoint stuff */
>  struct bpt {
>  	unsigned long	address;
> -	unsigned int	instr[2];
> +	/* Prefixed instructions can not cross 64-byte boundaries */
> +	unsigned int	instr[3] __aligned(64);

This is pretty wild, I didn't realize xmon executes breakpoints out
of line like this.

IMO the break point entries here should correspond with a range of
reserved bytes in .text so we patch instructions into normal executable
pages rather than .data.

Anyway that's for patch.

Thanks,
Nick
=