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Re: [Qemu-devel] softmmu thoughts

From: Magnus Damm
Subject: Re: [Qemu-devel] softmmu thoughts
Date: Wed, 20 Oct 2004 19:10:48 +0200


> - It would be more efficient and simpler to map one 4KB host memory page 
> every 8 KB. Then you can have a fixed mmap() mapping (no syscall 
> overhead to change the mappings) and a simple way to handle unaligned 
> accesses.

If I understand correctly the second page is used to generate a signal
for memory accesses that crosses the page boundary like I said earlier,
right? So the entire memory available to the guest is mapped into memory
using this technique. But how will protection work? What if two virtual
pages with different protection modes map to the same physical page? Do
we have two copies of all memory, one read-only and one read-write?

> - The critical point would be to keep standard soft MMU accesses for 
> device access. An architectural change is needed to do that, but it 
> seems easy to add.

How do you plan to do that? Just curious.

> - This patch should work with qemu, not qemu-fast. The future of 
> qemu-fast is to use a kernel module to have near native performances. It 
> is not worthwhile to invest time in soft MMU or dynamic translation when 
> you can just execute the code as is !

I fully agree. So, who is porting the patch from qemu-fast to qemu? I'm
available for some powerpc-specific hacking on the weekends.

/ magnus

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