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Re: [Xen-devel] [PATCH] paging_enabled and non-HVM guests

To: Simon Kagstrom <simon.kagstrom@xxxxxx>
Subject: Re: [Xen-devel] [PATCH] paging_enabled and non-HVM guests
From: Hollis Blanchard <hollisb@xxxxxxxxxx>
Date: Wed, 10 May 2006 09:51:54 -0500
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On Wed, 2006-05-10 at 08:06 +0200, Simon Kagstrom wrote:
> I won't argue for an incorrect fix, but as the code is right now it
> segmentation faults because the virtual address passed to
>         page = page_array[va >> PAGE_SHIFT] << PAGE_SHIFT;
> (with libxc incorrectly believing paging is disabled) accesses outside
> of page_array. I'll keep the patch privately for now since gdbserver
> breaks without it. 

Yes, and the reason is that page_array is supposed to be indexed with
*physical* addresses. The current code thinks that paging is disabled
(because CR0 is bad), assumes a virtual address is physical, and tries
to index into the array with it.

Pretty much every use of page_array needs to be abstracted so that it
does the right thing on both HVM and normal guests (normal guests will
have machine addresses in its page tables; HVM guests will have
physical). It's very unfortunate that the people who worked on this code
seem not to have tested or even thought about paravirtualized guests.

Hollis Blanchard
IBM Linux Technology Center

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