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From: Jordan Niethe <jniethe5@gmail.com>
Date: Tue, 24 Mar 2020 13:54:27 +1100
Message-ID: <CACzsE9qrNpfvoLKfdeXths4rKJ8jQcUic3=dFZ57ntogdeaMug@mail.gmail.com>
Subject: Re: [PATCH v4 00/16] Initial Prefixed Instruction support
To: Nicholas Piggin <npiggin@gmail.com>
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Cc: Alistair Popple <alistair@popple.id.au>,
 Balamuruhan S <bala24@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org,
 Daniel Axtens <dja@axtens.net>
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On Mon, Mar 23, 2020 at 9:21 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> Jordan Niethe's on March 23, 2020 7:25 pm:
> > On Mon, Mar 23, 2020 at 5:22 PM Nicholas Piggin <npiggin@gmail.com> wrote:
> >>
> >> Jordan Niethe's on March 20, 2020 3:17 pm:
> >> > A future revision of the ISA will introduce prefixed instructions. A
> >> > prefixed instruction is composed of a 4-byte prefix followed by a
> >> > 4-byte suffix.
> >> >
> >> > All prefixes have the major opcode 1. A prefix will never be a valid
> >> > word instruction. A suffix may be an existing word instruction or a
> >> > new instruction.
> >> >
> >> > This series enables prefixed instructions and extends the instruction
> >> > emulation to support them. Then the places where prefixed instructions
> >> > might need to be emulated are updated.
> >> >
> >> > The series is based on top of:
> >> > https://patchwork.ozlabs.org/patch/1232619/ as this will effect
> >> > kprobes.
> >> >
> >> > v4 is based on feedback from Nick Piggins, Christophe Leroy and Daniel Axtens.
> >> > The major changes:
> >> >     - Move xmon breakpoints from data section to text section
> >> >     - Introduce a data type for instructions on powerpc
> >>
> >> Thanks for doing this, looks like a lot of work, I hope it works out :)
> >>
> > Yes it did end up touching a lot of places. I started thinking that
> > that maybe it would be simpler to just use a u64 instead of the struct
> > for  instructions.
> > If we always keep the word instruction / prefix in the lower bytes,
> > all of the current masking should still work and we can use operators
> > again instead of ppc_inst_equal(), etc.
>
> Yeah.. I think now that you've done it, I prefer it this way.
Sorry, just to be clear which way do you mean?
>
> > It also makes printing easier. We could just #define INST_FMT %llx or
> > #define INST_FMT %x on powerpc32 and use that for printing out
> > instructions.
>
> Well, not sure about that. Would it make endian concerns more
> complicated? Print format for prefix might be '%016llx', but we
> don't want that for all instructions only prefixed ones, and I
> don't know if that is the way to go either.
Hm yeah that is true.
>
> We'll want to adopt some convention for displaying prefixed
> instruction bytes, but I don't know what what works best. I wonder
> if binutils or any userspace tools have a convention.
binutils-gdb upstream has supports disassembling prefixed instructions.
Here is what objdump looks like:
  44:    00 00 00 60     nop
  48:    00 00 00 07     pnop
  4c:    00 00 00 00
  50:    01 00 20 39     li      r9,1
  54:    00 00 00 06     paddi   r4,r9,3
  58:    03 00 89 38
  5c:    00 00 62 3c     addis   r3,r2,0
>
> Which reminds me, you might have missed show_instructions()?
> Although maybe you don't need that until we start using them in
> the kernel.
You are right I missed that here.
>
> Thanks,
> Nick