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Re: [Xen-devel] HYBRID: max_mapped_pfn in ept_set_entry()

At 10:29 +0100 on 29 Mar (1333016954), Ian Campbell wrote:
> On Thu, 2012-03-29 at 01:08 +0100, Mukesh Rathor wrote:
> > I'm trying to figure the right thing to do here. Should I just change
> > the "gfn > p2m->max_mapped_pfn" in ept_get_entry() to check for
> > INVALID_MFN? I really shouldn't be adjusting max_mapped_pfn for MMIO
> > pages, right?
> I'm no expert on the p2m side of things but ept_get_entry says:
>     /* This pfn is higher than the highest the p2m map currently holds */
>     if ( gfn > p2m->max_mapped_pfn )
>         goto out;
> which suggests to me that this is just an optimisation (skipping a
> lookup which can never succeed) and therefore it is appropriate to
> update max_mapped_pfn. After all a 1-1 mapped pfn is still a pfn.

Yes, you should adjust max_mapped_pfn, in ept_set_entry() and in
p2m_set_entry() (p2m-pt.c) so we don't get the same bug reappearing
on AMD.  Instead of checking mfn_valid(), they should check 
(p2mt != p2m_invalid && p2mt != p2m_mmio_dm).

> Other places, like ept_walk_table seem to make a similar optimisation. 
> Unless there is some reason to assume that these functions will never be
> passed an MMIO pfn?

No - it's just that this check predates anything other than RAM and
emulated MMIO.


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