xen/arm64: head: Remove 1:1 mapping as soon as it is not used

xen/arm64: head: Remove 1:1 mapping as soon as it is not used

The 1:1 mapping may clash with other parts of the Xen virtual memory
layout. At the moment, Xen is handling the clash by only creating a
mapping to the runtime virtual address before enabling the MMU.

The rest of the mappings (such as the fixmap) will be mapped after the
MMU is enabled. However, the code doing the mapping is not safe as it
replace mapping without using the Break-Before-Make sequence.

As the 1:1 mapping can be anywhere in the memory, it is easier to remove
all the entries added as soon as the 1:1 mapping is not used rather than
adding the Break-Before-Make sequence everywhere.

It is difficult to track where exactly the 1:1 mapping was created
without a full rework of create_page_tables(). Instead, introduce a new
function remove_identity_mapping() will look where is the top-level entry
for the 1:1 mapping and remove it.

The new function is only called for the boot CPU. Secondary CPUs will
switch directly to the runtime page-tables so there are no need to
remove the 1:1 mapping. Note that this still doesn't make the Secondary
CPUs path safe but it is not making it worst.

Note that the TLB flush sequence is same sequence as described in
asm-arm/arm32/flushtlb.h with a twist. Per D5-2530 ARM DDI 0487D.a,
a dsb nsh is sufficient for local flush. This part of the Arm Arm
was missed while reworking the header and therefore a more conservative
way was adopted.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>