Re: [Qemu-devel] using mmap?

From: Mark Williamson <mark.williamson@cl.cam.ac.uk>
To: qemu-devel@nongnu.org
Cc: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] using mmap?
Date: Fri, 15 Dec 2006 16:32:06 +0000	[thread overview]
Message-ID: <200612151632.07145.mark.williamson@cl.cam.ac.uk> (raw)
In-Reply-To: <200612141645.40403.paul@codesourcery.com>

> I'm also doubtful how much benefit it gave in practice. I'm sure it would
> be good for synthetic CPU benchmarks. However using mmap significantly
> increases the overhead of context switches/tlb misses.
>
> To get good overall performance I suspect you're going to need closer
> cooperation with the kernel than mmap gives you. If you really want to make
> cross-emulation go fast I suggest working with the xen and/or kvm people to
> integrate qemu dynamic translation into those products. In theory I'd guess
> you should be able to plug it in as an alternative to the HVM code. I've no
> idea how close that is to being practical.

http://wiki.xensource.com/xenwiki/HVM/V2E

The v2e stuff allows execution state to be extracted from the real CPU, 
plugged into QEmu for a bit for emulation, then transferred back to the real 
CPU again.  This is initially to be used for supporting Big Real Mode 
emulation on HVM platforms.  Later on it's planned to be used to accelerate 
IO emulation further.

Eventually this may provide a means to use QEmu's translator to execute kernel 
code whilst running user mode code under Xen.  It may be that this isn't as 
fast as other approaches, but it'd be a useful feature for Xen to have IMO.

Cheers,
Mark

> > Wouldn't this be a *significant*
> > performance enhancement for this setup which I'm sure is a common one?
> > Maybe this can be implemented for regular processes on the guest and
> > only use the softmmu for the kernel?  Would someone point me in the
> > right direction for technical information?  I have had to switch to
> > vmware free player until Qemu+KQEMU reaches a point of similar
> > performance, but I would really rather see Qemu advance further.
>
> If you're using an accelerator (eg. kqemu or kvm) this is all irelevant as
> most code isn't run by qemu, it's virtualized by the accelerator. qemu just
> does the IO emulation.
>
> Paul
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!