xen/arm: Zero BSS after the MMU and D-cache is turned on

xen/arm: Zero BSS after the MMU and D-cache is turned on

At the moment BSS is zeroed before the MMU and D-Cache is turned on.
In other words, the cache will be bypassed when zeroing the BSS section.

On Arm64, per the Image protocol [1], the state of the cache for BSS region
is not known because it is not part of the "loaded kernel image".

On Arm32, the boot protocol [2] does not mention anything about the
state of the cache. Therefore, it should be assumed that it is not known
for BSS region.

This means that the cache will need to be invalidated twice for the BSS
region:
    1) Before zeroing to remove any dirty cache line. Otherwise they may
    get evicted while zeroing and therefore overriding the value.
    2) After zeroing to remove any cache line that may have been
    speculated. Otherwise when turning on MMU and D-Cache, the CPU may
    see old values.

At the moment, the only reason to have BSS zeroed early is because the
boot page tables are part of it. To avoid the two cache invalidations,
it would be better if the boot page tables are part of the "loaded
kernel image" and therefore be zeroed when loading the image into
memory. A good candidate is the section .data.page_aligned.

A new macro DEFINE_BOOT_PAGE_TABLE is introduced to create and mark
page-tables used before BSS is zeroed. This includes all boot_* but also
xen_fixmap as zero_bss() will print a message when earlyprintk is
enabled.

[1] linux/Documentation/arm64/booting.txt
[2] linux/Documentation/arm/Booting

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