xen/page_alloc: relax the BUILD_BUG_ON() in xenheap_max_mfn()

In the near future, we are considering to use a common xen/domain heap for
Arm 32-bit V8-R. In this setup, both PADDR_BITS and BITS_PER_LONG will be
32. Therefore, the condition PADDR_BITS >= BITS_PER_LONG will become true.

Looking at the commit that introduce the BUILD_BUG_ON (88e3ed6164
"x86/NUMA: make init_node_heap() respect Xen heap limit") and the current
use, it seems this check was mainly to ensure that the shift of xenheap_bits
is not going to be undefined (>> N for a N-bit type is undefined).

So far, all the shifts are using "xenheap_bits - PAGE_SHIFT". Therefore, the
existing code should work for 32-bit architecture as long as the result of
the substraction is below 32.

Therefore relax the BUILD_BUG_ON() to check that (PADDR_BITS - PAGE_SHIFT)
is not equal of above BITS_PER_LONG.

Note that we would need further precaution if we ended up to use
'xenheap_bits' alone in shifts.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Signed-off-by: Julien Grall <julien@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/common/page_alloc.c

@@ -2245,7 +2245,7 @@ void __init xenheap_max_mfn(unsigned long mfn)
 {
     ASSERT(!first_node_initialised);
     ASSERT(!xenheap_bits);
-    BUILD_BUG_ON(PADDR_BITS >= BITS_PER_LONG);
+    BUILD_BUG_ON((PADDR_BITS - PAGE_SHIFT) >= BITS_PER_LONG);
     xenheap_bits = min(flsl(mfn + 1) - 1 + PAGE_SHIFT, PADDR_BITS);
     printk(XENLOG_INFO "Xen heap: %u bits\n", xenheap_bits);
 }