[XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator

[Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>]

We want to ensure that "xenheap_bits - PAGE_SHIFT" is strictly less than
the number of bits to represent unsigned long as it is used a rhs operand
to shift mfn.
It is also important to note that the "xenheap_bits - PAGE_SHIFT" is never
used to shift an address (rather than a frame number), and going forward
then also shouldn't be (perhaps unless further precautions are taken).

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
---

Currently this change will not have any impact on the existing architectures.
The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs

------------------------------------------------
| Arch      |   PADDR_BITS    |   BITS_PER_LONG |
------------------------------------------------
| Arm_64    |   48            |   64            |
| Arm_32    |   40            |   32            |
| RISCV_64  |   Don't know    |   64            |
| x86       |   52            |   64            |
-------------------------------------------------

However, this will change when we introduce a platform (For eg Cortex-R52) which
supports 32 bit physical address and BITS_PER_LONG. This platform does not 
follow
the same code path as Arm_32.
Thus, I have introduced this change as I don't see it causing a regression on
any of the supported platforms.

Changes from v1:-
1. Changed the check from "(PADDR_BITS > BITS_PER_LONG)" to "((PADDR_BITS - 
PAGE_SHIFT) >= BITS_PER_LONG)"
2. Updated the commit message to explain the reason for this.

v2 :-
1. Updated the commit message.

 xen/common/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 62afb07bc6..c5b8c7444f 100644
--- a/xen/common/page_alloc.c
+++ b/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);
 }
-- 
2.17.1