KVM: PPC: Book3S HV: Fix KSM memory corruption

Testing with KSM active in the host showed occasional corruption of
guest memory.  Typically a page that should have contained zeroes
would contain values that look like the contents of a user process
stack (values such as 0x0000_3fff_xxxx_xxx).

Code inspection in kvmppc_h_protect revealed that there was a race
condition with the possibility of granting write access to a page
which is read-only in the host page tables.  The code attempts to keep
the host mapping read-only if the host userspace PTE is read-only, but
if that PTE had been temporarily made invalid for any reason, the
read-only check would not trigger and the host HPTE could end up
read-write.  Examination of the guest HPT in the failure situation
revealed that there were indeed shared pages which should have been
read-only that were mapped read-write.

To close this race, we don't let a page go from being read-only to
being read-write, as far as the real HPTE mapping the page is
concerned (the guest view can go to read-write, but the actual mapping
stays read-only).  When the guest tries to write to the page, we take
an HDSI and let kvmppc_book3s_hv_page_fault take care of providing a
writable HPTE for the page.

This eliminates the occasional corruption of shared pages
that was previously seen with KSM active.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

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