xen/gnttab: Store frame GFN in struct page_info on Arm

xen/gnttab: Store frame GFN in struct page_info on Arm

Rework Arm implementation to store grant table frame GFN
in struct page_info directly instead of keeping it in
standalone status/shared arrays. This patch is based on
the assumption that a grant table page is a xenheap page.

To cover 64-bit/40-bit IPA on Arm64/Arm32 we need the space
to hold 52-bit/28-bit + extra bit value respectively. In order
to not grow the size of struct page_info borrow the required
amount of bits from type_info's count portion which current
context won't suffer (currently only 1 bit is used on Arm).
Please note, to minimize code changes and avoid introducing
an extra #ifdef-s to the header, we keep the same amount of
bits on both subarches, although the count portion on Arm64
could be wider, so we waste some bits here.

Introduce corresponding PGT_* constructs and access macro
page_get(set)_xenheap_gfn. Please note, all accesses to
the GFN portion of type_info field should always be protected
by the P2M lock. In case when it is not feasible to satisfy
that requirement (risk of deadlock, lock inversion, etc)
it is important to make sure that all non-protected updates
to this field are atomic.
As several non-protected read accesses still exist within
current code (most calls to page_get_xenheap_gfn() are not
protected by the P2M lock) the subsequent patch will introduce
hardening code for p2m_remove_mapping() to be called with P2M
lock held in order to check any difference between what is
already mapped and what is requested to be ummapped.

Update existing gnttab macros to deal with GFN value according
to new location. Also update the use of count portion of type_info
field on Arm in share_xen_page_with_guest().

While at it, extend this simplified M2P-like approach for any
xenheap pages which are proccessed in xenmem_add_to_physmap_one()
except foreign ones. Update the code to set GFN portion after
establishing new mapping for the xenheap page in said function
and to clean GFN portion when putting a reference on that page
in p2m_put_l3_page().

And for everything to work correctly introduce arch-specific
initialization pattern PGT_TYPE_INFO_INITIALIZER to be applied
to type_info field during initialization at alloc_heap_pages()
and acquire_staticmem_pages(). The pattern's purpose on Arm
is to clear the GFN portion before use, on x86 it is just
a stub.

This patch is intended to fix the potential issue on Arm
which might happen when remapping grant-table frame.
A guest (or the toolstack) will unmap the grant-table frame
using XENMEM_remove_physmap. This is a generic hypercall,
so on x86, we are relying on the fact the M2P entry will
be cleared on removal. For architecture without the M2P,
the GFN would still be present in the grant frame/status
array. So on the next call to map the page, we will end up to
request the P2M to remove whatever mapping was the given GFN.
This could well be another mapping.

Please note, this patch also changes the behavior how the shared_info
page (which is xenheap RAM page) is mapped in xenmem_add_to_physmap_one().
Now, we only allow to map the shared_info at once. The subsequent
attempts to map it will result in -EBUSY. Doing that we mandate
the caller to first unmap the page before mapping it again. This is
to prevent Xen creating an unwanted hole in the P2M. For instance,
this could happen if the firmware stole a RAM address for mapping
the shared_info page into but forgot to unmap it afterwards.

Besides that, this patch simplifies arch code on Arm by
removing arrays and corresponding management code and
as the result gnttab_init_arch/gnttab_destroy_arch helpers
and struct grant_table_arch become useless and can be
dropped globally.

Suggested-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>