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Re: [Qemu-devel] Potential to accelerate QEMU for specific architectures

From: Andreas Färber
Subject: Re: [Qemu-devel] Potential to accelerate QEMU for specific architectures
Date: Sat, 25 May 2013 21:06:50 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130329 Thunderbird/17.0.5


Am 24.05.2013 21:24, schrieb Lior Vernia:
> I am running x86 applications on an ARM device using QEMU, and found
> it too slow for my needs.

Before we start going into technical details, what are you trying to
achieve on a high level and how did you try to do it?

Are you using qemu-system-x86_64 or qemu-x86_64? The latest v1.5.0?

> This is to be expected, of course, this is
> not a complaint.

Especially since most people still run on x86 ...

> However, I was wondering whether this could be helped
> by "overriding" the generic binary translation mechanism and focusing
> on lower level binary translation just from x86 to ARM.
> It's clear to me that this isn't a small project, but it might be
> important enough for me to invest myself in. However, before I jump
> into it, I wanted to inquire whether this would be worthwhile at all.
> Does anyone have any estimate as to how big of a gain that could
> achieve? Or whether a more significant improvement could be achieved
> by further tweaking that didn't occur to me?

... the tcg/arm/ code does not get a lot of love, so you might be able
to squeeze some more performance out of it by implementing optional TCG
ops or optimizing existing implementations. In theory most TCG ops
should correspond to a machine instruction (where available); there's a
TCG-level optimizer to create more efficient code, but it's a tradeoff
between time for code optimization and execution time.

Needless to say that you should enable -O3 optimization (or something)
for the core C code and not to enable debug features in configure for
your performance measurements. :)

Whatever implementation you experiment with, get familiar with our
Git-based workflow and try to stay close to qemu.git code or otherwise
you'll create a fork with little chance of getting integrated into the
code base - meaning both we don't get your speedups and you don't get
our latest features and bugfixes. One such example was the attempt to
use LLVM instead of TCG.


> Proper disclosure: I'm fairly new to this whole cross-architecture deal.
> Yours, Lior.

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