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From: =?utf-8?Q?Llu=C3=ADs_Vilanova?= <vilanova@ac.upc.edu>
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Date: Sat, 21 Oct 2017 17:05:46 +0300
In-Reply-To: <CAFEAcA8drKvtqmLNKx4oQs+zGUNBgnh3g9Q1X=TbPzoqT2iC2g@mail.gmail.com>
	(Peter Maydell's message of "Sun, 15 Oct 2017 17:47:21 +0100")
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Subject: Re: [Qemu-devel] [PATCH v6 01/22] instrument: Add documentation
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Markus Armbruster <armbru@redhat.com>

Peter Maydell writes:

> On 15 October 2017 at 17:30, Llu=C3=ADs Vilanova <vilanova@ac.upc.edu> wr=
ote:
>> Thinking about it, shouldn't this always be the same given QEMU's TLB/pa=
ge table
>> consistency assurances?

> What TLB/page table consistency assurances? For ARM at least
> we will only update (ie flush) the TLB when the guest next
> executes a relevant TLB maintenance instruction. So a
> misbehaving guest can set things up so the page table
> is completely different from what's in QEMU's TLB if it
> wants. This all falls in the realms of architecturally
> unpredictable behaviour for the guest -- whether you
> want the instrumentation to be confused as well is a
> different question...

I meant that if the contents of a virtual memory page change while QEMU is
translating an instruction, it must be able to detect that and act accordin=
gly
for correctness.

Having that in mind, the same should hold true when an instrumentor reads a
page's contents during translation (e.g., to gather information on opcodes).


Cheers,
  Lluis