x86: don't flush the whole cache when changing cachability

x86: don't flush the whole cache when changing cachability

Introduce the FLUSH_VA_VALID flag to flush_area_mask() and friends to
say that it is safe to use CLFLUSH (i.e., the virtual address is still
valid).

Use this when changing the cachability of the Xen direct mappings (in
response to the guest changing the cachability of its mappings). This
significantly improves performance by avoiding an expensive WBINVD.

This fixes a performance regression introduced by
c61a6f74f80eb36ed83a82f713db3143159b9009 (x86: enforce consistent
cachability of MMIO mappings), the fix for XSA-154.

e.g., A set_memory_wc() call in Linux:

before: 4097 us
after:    47 us

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>