arm/xen: Add misuse warning to virt_to_gfn

As virt_to_gfn uses virt_to_phys, it will return invalid addresses when
used with vmalloc'd addresses. This patch introduces a warning, when
virt_to_gfn is used in this way.

Signed-off-by: Simon Leiner <simon@leiner.me>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/20200825093153.35500-2-simon@leiner.me
Signed-off-by: Juergen Gross <jgross@suse.com>

include/xen/arm/page.h

@@ -76,7 +76,11 @@ static inline unsigned long bfn_to_pfn(unsigned long bfn)
 #define bfn_to_local_pfn(bfn)	bfn_to_pfn(bfn)
 
 /* VIRT <-> GUEST conversion */
-#define virt_to_gfn(v)		(pfn_to_gfn(virt_to_phys(v) >> XEN_PAGE_SHIFT))
+#define virt_to_gfn(v)                                                         \
+	({                                                                     \
+		WARN_ON_ONCE(!virt_addr_valid(v));                              \
+		pfn_to_gfn(virt_to_phys(v) >> XEN_PAGE_SHIFT);                 \
+	})
 #define gfn_to_virt(m)		(__va(gfn_to_pfn(m) << XEN_PAGE_SHIFT))
 
 /* Only used in PV code. But ARM guests are always HVM. */