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Re: [Qemu-devel] qemu-fast kernel patch question

From: Jim C. Brown
Subject: Re: [Qemu-devel] qemu-fast kernel patch question
Date: Sun, 25 Jul 2004 16:03:11 -0400
User-agent: Mutt/1.4i

On Sun, Jul 25, 2004 at 12:24:16PM +0200, Johannes Martin wrote:
> Hi,
> Here's a quote from qemu-tech.html:
>   ---
>   2.10 MMU emulation
>   For system emulation, QEMU uses the mmap() system call to emulate the
>   target CPU MMU. It works as long the emulated OS does not use an area
>   reserved by the host OS (such as the area above 0xc0000000 on x86
>   Linux).
>   ---
> As far as I understand this, the qemu kernel patch makes the guest kernel
> use a different reserved area than the linux host. So it shouldn't matter
> which kernel I apply the patch to, right? If I patch the (linux) host
> kernel, I should be able to boot any unpatched (linux) guest kernel, if I
> don't patch the (linux) host kernel, I have to patch my (linux) guest
> kernels. Or do we need a patch-aware qemy-fast to run on a patched host?
> Assuming that OS XYZ does not use the area around 0xc0000000, it should
> work inside qemu-fast, right? And if it does use that area, it should work
> inside qemu-fast on a patched host?
> Maybe one of the wizards could clarify this...
> Thanks
>       Johannes

You could patch the host not to use that area, but then it would have to use
another area ... thus you'd have to patch the guest anyways, regardless of the

Fabrice has said that the eventual idea is to have qemu-fast detect which areas
are not accessable via mmap() and use the softmmu to emulate for only those
areas. The rest would be accessed via the faster mmap(). So, if you patch
qemu-fast in the right way, you might not have to patch the guest or the host. 

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