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Re: [Qemu-discuss] QEMU: DBT vs. Interpretation

From: Peter Maydell
Subject: Re: [Qemu-discuss] QEMU: DBT vs. Interpretation
Date: Wed, 28 Jan 2015 10:18:25 +0000

On 28 January 2015 at 02:51, Dale R. Worley <address@hidden> wrote:
> Javier Picorel <address@hidden> writes:
>> We are trying to make QEMU deterministic for
>> architectural simulation. In the absence of I/O,
>> let's say only user code or exceptions, is there
>> any source of indeterminism (e.g., non deterministic
>> compiler optimizations, TB indeterminism) of
>> QEMU's DBT versus a canonical interpreter? Thanks!
> I'm not sure exactly what information you're trying to obtain.  Given
> that most CPU architectures have multiple implementations, and many of
> those have complex internal operations, it's difficult to make an
> emulator deterministic in any way other than "its behavior conforms to
> the architecture specification".

I had assumed the meaning here was "deterministic" in the sense
of "every time you run QEMU with the same binary and settings
you get exactly the same execution run". With the default
switches we don't provide that (we give 'best performance'
instead). -icount is I think supposed to be deterministic but
probably buggy. Determinism is a requirement for the checkpoint/
reverse execution work so the people doing that are probably
best placed to say what the current status is.

-- PMM

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