Suraj Jitindar Singh [off-list ref] writes:

On Fri, 2019-07-12 at 23:09 +1000, Michael Ellerman wrote:

quoted

Suraj Jitindar Singh [off-list ref] writes:

quoted

The virtual real mode addressing (VRMA) mechanism is used when a
partition is using HPT (Hash Page Table) translation and performs
real mode accesses (MSR[IR|DR] = 0) in non-hypervisor mode. In this
mode effective address bits 0:23 are treated as zero (i.e. the
access
is aliased to 0) and the access is performed using an implicit 1TB
SLB
entry.

The size of the RMA (Real Memory Area) is communicated to the guest
as
the size of the first memory region in the device tree. And because
of
the mechanism described above can be expected to not exceed 1TB. In
the
event that the host erroneously represents the RMA as being larger
than
1TB, guest accesses in real mode to memory addresses above 1TB will
be
aliased down to below 1TB. This means that a memory access
performed in
real mode may differ to one performed in virtual mode for the same
memory
address, which would likely have unintended consequences.

To avoid this outcome have the guest explicitly limit the size of
the
RMA to the current maximum, which is 1TB. This means that even if
the
first memory block is larger than 1TB, only the first 1TB should be
accessed in real mode.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>

I added:

Fixes: c3ab300ea555 ("powerpc: Add POWER9 cputable entry")
Cc: stable@vger.kernel.org # v4.6+


Which is not exactly correct, but probably good enough?

I think we actually want:
Fixes: c610d65c0ad0 ("powerpc/pseries: lift RTAS limit for hash")

Which is what actually caused it to break and for the issue to present
itself.

Thanks, I used that instead.

cheers