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Date: Mon, 26 Nov 2018 14:07:33 -0500
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20181126190733.GC6688@flamenco>
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	<87lg5f51sz.fsf@linaro.org>
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Subject: Re: [Qemu-devel] [RFC 23/48] translator: add plugin_insn argument
 to translate_insn
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To: Richard Henderson <richard.henderson@linaro.org>
Cc: Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Stefan Hajnoczi <stefanha@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org, Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>, =?iso-8859-1?Q?Llu=EDs?= Vilanova <vilanova@ac.upc.edu>

On Mon, Nov 26, 2018 at 10:27:12 -0800, Richard Henderson wrote:
> On 11/26/18 6:52 AM, Alex Bennée wrote:
> > I'm not convinced this is the best way to go about it. We end up having
> > to sprinkle the plugin calls into each decoder rather than keeping all
> > the infrastructure in the common main loop. However the common loop will
> > need to know the total number of bytes decoded so we could change the
> > declaration to:
> > 
> >   int (*translate_insn)(DisasContextBase *db, CPUState *cpu);
> > 
> > and return the number of bytes decoded. 
> 
> Returning the number of bytes is more difficult than simply just
> 
>     old_pc = db->pc_next;
>     opc->translate_insn(db, cpu);
>     bytes = db->pc_next - old_pc;
> 
> requiring no target changes at all.

The main reason why I added the qemu_plugin_insn_append calls
was to avoid reading the instructions twice from guest memory,
because I was worried that doing so might somehow alter the
guest's execution, e.g. what if we read a cross-page instruction,
and both pages mapped to the same TLB entry? We'd end up having
more TLB misses because instrumentation was enabled.

If you think that's not really a concern, we could just re-do
the reads in the translator loop and get the size as above.

Thanks,

		Emilio