[Xen-devel] bogus gfn - mfn - gfn - mfn checks in guest_physmap_add_entry

[Olaf Hering <olaf@xxxxxxxxx>]

Hello,

what is the purpose of the mfn_to_gfn() check in
guest_physmap_add_entry()?
This function gets a fresh mfn and its gfn passed to update the global
p2m state. But before doing that, it checks wether that fresh mfn maps
still to some gfn. If it does, it checks if that gfn maps to any mfn. If
it doesnt, Xen crashes with an assert.

Now, if that mfn was part of an old guest, should that old guest cleanup
all of its mfns and update the machine_to_phys_mapping[]? Appearently
that rarely happens.
And if there is still some random gfn number in that array, the function
tries to see what happens with this number in the current guests
context. IF that number happens to be a gfn in paged-out state, there
will be no mfn. So the ASSERT triggers.

I would guess that if guest_physmap_add_entry() gets a page with type
p2m_ram_rw, nothing else can own that page. Is that right?
If so, this ASSERT or most of the loop can be removed.


Olaf


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