Re: [Xen-devel] PAE xen + linux kernel boots ...

[Scott Parish <srparish@xxxxxxxxxx>]

On Sat, Apr 30, 2005 at 09:01:17AM +0000, Scott Parish wrote:

> On Wed, Apr 27, 2005 at 02:03:17PM +0200, Gerd Knorr wrote:
> 
> > Well, now it does, boots up to a login prompt ;)
> 
> pl1e would straddle a page boundary

I swear there is a muse associated with the send button on email
clients.

In this case the epiphany was the obvious--the problem was that we're
missing alignment. But why?

On the linux side of things we have the following in pgtable-3level.h:

   #if 1 /* writable pagetables */
   static inline void set_pte(pte_t *ptep, pte_t pte)
   {
            ptep->pte_high = pte.pte_high;
            smp_wmb();
            ptep->pte_low = pte.pte_low;
   }
   ...

Here's what (i'm thinking) is going on. We go to set the high bits
(first for atomicy: we don't set the active bit till last), but take
a page fault, on the high bits--a 4 byte offset.

Switch to xen, which is going to emulate some instructions and fake
the writing. We eventually end up in ptwr_emulated_update(), who among
other things, tries to copy the full l1_pgentry_t (64bits), but from
the 4 byte offset, that is the 4 high bytes and then 4 bytes of
undefined memory that may even be in another page.

sRp

-- 
Scott Parish

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