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From: Jordan Niethe <jniethe5@gmail.com>
Date: Thu, 27 Feb 2020 11:11:47 +1100
Message-ID: <CACzsE9p4siTRgCnC6GPSn+89SnPr5CTTBk5+LOfS8BX+1OmMZg@mail.gmail.com>
Subject: Re: [PATCH v3 09/14] powerpc/xmon: Add initial support for prefixed
 instructions
To: Nicholas Piggin <npiggin@gmail.com>
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Cc: Alistair Popple <alistair@popple.id.au>, Daniel Axtens <dja@axtens.net>,
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On Wed, Feb 26, 2020 at 6:10 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> 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.
> >
> > 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.
> >
> > 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.
> >
> > No support for disassembling prefixed instructions.
> >
> > 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(-)
> >
> > 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.
Would it make sense to use vmalloc_exec() and use that like we are
going to do in kprobes()?
>
> Anyway that's for patch.
>
> Thanks,
> Nick