[Xen-devel] Re: [PATCH 05/12] xen: add m2p override mechanism
On Mon, 10 Jan 2011, Konrad Rzeszutek Wilk wrote:
> > @@ -83,6 +85,14 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
> > */
> > __get_user(pfn, &machine_to_phys_mapping[mfn]);
> > + /*
> > + * If this appears to be a foreign mfn (because the pfn
> > + * doesn't map back to the mfn), then check the local override
> > + * table to see if there's a better pfn to use.
> > + */
> > + if (get_phys_to_machine(pfn) != mfn)
> Any reason to not use 'pfn_to_mfn' call instead? If we do want
> to use the get_hhys_to_machine wouldn't be easier to just
> check for 'FOREIGN_FRAME_BIT' set? Or is just mostly a copy-n-paste
> of mfn_to_local_pfn?
pfn in this case is the return value of m2p(mfn); there are 3 possible
1) the mfn was a regular ram page belonging to the domain so
get_phys_to_machine(pfn) == mfn. Nothing to see here.
2) the mfn was a granted page so pfn is actually the pfn in the source
domain and doesn't mean anything in our domain.
In this case get_phys_to_machine(pfn) is always different from mfn,
either because the mapping in our domain is just different or because
the FOREIGN_FRAME_BIT is set.
3) the pfn belongs to a 1:1 region. In this case
get_phys_to_machine(pfn) != mfn because of the IDENTITY_FRAME_BIT but we
don't actually want to look in the m2p_override because we are not going
to find anything useful there.
Considering all this, I propose this change:
if (get_phys_to_machine(pfn) != mfn)
if (pfn_to_mfn(pfn) != mfn)
- it won't make any differences in case 1);
- it will avoid false potives in case 3);
- in case 2) if p2m(pfn) => mfn in both the source and the destination
domain then we might skip the check in the m2p_override but we don't
care because by luck we got the correct value anyway (m2p(mfn) ==
However I understand that it might be subtle and needs a very long
comment so a simpler alternative would be to explicitly remove the
IDENTITY_FRAME_BIT from the check:
if ((get_phys_to_machine(pfn) & ~IDENTITY_FRAME_BIT) != mfn)
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