xen/arm: mm: flush_page_to_ram() only need to clean to PoC

xen/arm: mm: flush_page_to_ram() only need to clean to PoC

At the moment, flush_page_to_ram() is both cleaning and invalidate to
PoC the page.

The goal of flush_page_to_ram() is to prevent corruption when the guest
has disabled the cache (the cache line may be dirty) and the guest to
read previous content.

Per this definition, the invalidating the line is not necessary. So
invalidating the cache is unnecessary. In fact, it may be counter-
productive as the line may be (speculatively) accessed a bit after.
So this will incurr an expensive access to the memory.

More generally, we should avoid interferring too much with cache.
Therefore, flush_page_to_ram() is updated to only clean to PoC the page.

The performance impact of this change will depend on your
workload/processor.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>