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[Xen-devel] Re: Are linker scripts needed?

On 23 May 2005, at 07:00, Arun Sharma wrote:

Yes, we do need linker scripts to support VMX guests with large memory (> 256MB). Without the linker scripts, we found instabilities with a 512MB guest.

The reason for this is, qemu is mapping all of guest physical in it's address space. The linker script moves all of the qemu text/data/stack to the upper part of the user virtual to leave a large enough hole for mapping guest physical using xc_map_foreign.

I suppose it improves performance and simplifies the data path, but it should probably dynamically map the vmx guest's pages for I/O on demand. After all, this approach isn't going to work for bigmem domains on i386 (particularly once PAE is working). So it seems a bad idea to come to rely on it any more than as a bit of a hack to get you up and running.

Best of all (imo) would be for it to act as a 'driver in the middle': connecting the fully virt guest through to an ordinary backend driver, and not itself touching I/O buffers at all. Perhaps ultimately we can push the qemu drivers into the vmx container itself, perhaps in 'virtual smm mode', so that from outside the container it looks just like any other paravirt I/O channel.

Our intention is to use qemu for i386, x86-64 and ia64. The "target-i386-dm" is a misnomer and inherited from upstream. I should probably kill the concept of a "target architecture" since ioemu is cpu architecture independent right now.

Sounds good.

 -- Keir

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