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Re: [Qemu-devel] RFC: Why does target/m68k RTE insn. use gen_exception

From: Alex Bennée
Subject: Re: [Qemu-devel] RFC: Why does target/m68k RTE insn. use gen_exception
Date: Wed, 10 Jul 2019 19:15:50 +0100
User-agent: mu4e 1.3.2; emacs 26.1

Lucien Murray-Pitts <address@hidden> writes:

>> On Wed, Jul 10, 2019 at 4:04 AM Richard Henderson <
> address@hidden> wrote:
>> > I did have a suggestion.  It was fairly detailed.
>> > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg06522.html
>> Your solution is elegant at about 10 lines that return getl_ilen(pc), but
> it seems the s390 has a far simpler
> instruction word format than the m68k.
> However then that got me to thinking, it seems that we can call a portion
> of the TCG system to disassemble a single instruction.
>     TranslationBlock tb;
>     tb.pc = env->pc;
>     gen_intermediate_code(cs, &tb, /* max isn */ 1);
>     int ilen = tb.size;
>     printf( "PC: %08x sz:%08x\n", env->pc, tb, ilen ) ;
> I am very new to TCG, so it does seem there is a lot of code in the
> translator_loop that appears to be interacting with the CPU model/state.
> Should I be worried about this, or is this a safe function to call outside
> of the translator core proper?

I would recommend against it - the time to do stuff like this would be
during translation phase where you can save the data. Don't re-invoke the
translator while trying to process an exception.

Is the instruction format that irregular that you can't do a simple
disassembly in a helper?

> (if everyone is too busy I can dig by myself but I think its going to take
> some time)
> Cheers,
> Luc

Alex Bennée

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