Re: PAE issue (32-on-64 work)

["Bruce Rogers" <BROGERS@novell.com>]

NetWare is also a PAE guest, but doesn't put anything in the rest of the page, so
zapping would be fine for NetWare.

- Bruce

>>> On 10/19/2006 at 6:56 AM, in message <C15D34A3.2CB1%Keir.Fraser@cl.cam.ac.uk>,
Keir Fraser <Keir.Fraser@cl.cam.ac.uk> wrote:
> On 19/10/06 11:39, "Jan Beulich" <jbeulich@novell.com> wrote:
> 
>> Just now I found that there is a resulting issue for the 32on64 work I'm
>> doing: Since none of the entries 4...511 of the PMD get initialized in 
> Linux,
>> and since Xen nevertheless has to validate all 512 entries (in order to
>> avoid making available translations that could be used during speculative
>> execution), the validation has the potential to fail (and does in reality),
>> resulting in the guest dying. The only option I presently see is to special
>> case the compatibility guest in the l3 handling and (I really hate to do
>> that) clear out the 518 supposedly unused entries (or at least clear
>> their present bits), meaning that no guest may ever make clever
>> assumptions and try to store some other data in the unused portion of
>> the pgd page.
> 
> Either copy the PGDs out into a shadow L3, as we do for PAE Xen today. Or,
> as you say, zap the 508 unused entries. No guest uses them -- I'm pretty
> sure Linux is the only PAE-capable guest (others are non-pae or 64-bit).
> Storing other stuff in the page would be inconvenient anyway since it has to
> be read-only.
> 
>  -- Keir
> 
> 
> 
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