Re: [PATCH] powerpc: mm: Limit rma_size to 1TB when running without HV mode

On Wed, 2019-07-10 at 05:20:18 UTC, Suraj Jitindar Singh wrote:

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>
Tested-by: Satheesh Rajendran <redacted>
Reviewed-by: David Gibson <redacted>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/da0ef93310e67ae6902efded60b6724dab27a5d1

cheers