[PATCH] memory: fix build with COVERAGE but !HVM

[Jan Beulich <jbeulich@xxxxxxxx>]

Xen is heavily relying on the DCE stage to remove unused code so the
linker doesn't throw an error because a function is not implemented
yet we defined a prototype for it.

On some GCC versions (such as 9.4 provided by Debian sid), the compiler
DCE stage will not manage to figure that out for
xenmem_add_to_physmap_batch():

ld: ld: prelink.o: in function `xenmem_add_to_physmap_batch':
/xen/xen/common/memory.c:942: undefined reference to `xenmem_add_to_physmap_one'
/xen/xen/common/memory.c:942:(.text+0x22145): relocation truncated
to fit: R_X86_64_PLT32 against undefined symbol `xenmem_add_to_physmap_one'
prelink-efi.o: in function `xenmem_add_to_physmap_batch':
/xen/xen/common/memory.c:942: undefined reference to `xenmem_add_to_physmap_one'
make[2]: *** [Makefile:215: /root/xen/xen/xen.efi] Error 1
make[2]: *** Waiting for unfinished jobs....
ld: /xen/xen/.xen-syms.0: hidden symbol `xenmem_add_to_physmap_one' isn't 
defined
ld: final link failed: bad value

It is not entirely clear why the compiler DCE is not detecting the
unused code. However, cloning the check introduced by the commit below
into xenmem_add_to_physmap_batch() does the trick.

No functional change intended.

Fixes: d4f699a0df6c ("x86/mm: p2m_add_foreign() is HVM-only")
Reported-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
---
Julien, since I reused most of your patch'es description, I've kept your
S-o-b. Please let me know if you want me to drop it.

--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -904,6 +904,19 @@ static int xenmem_add_to_physmap_batch(s
 {
     union add_to_physmap_extra extra = {};
 
+    /*
+     * While, unlike xenmem_add_to_physmap(), this function is static, there
+     * still have been cases observed where xatp_permission_check(), invoked
+     * by our caller, doesn't lead to elimination of this entire function when
+     * the compile time evaluation of paging_mode_translate(d) is false. Guard
+     * against this be replicating the same check here.
+     */
+    if ( !paging_mode_translate(d) )
+    {
+        ASSERT_UNREACHABLE();
+        return -EACCES;
+    }
+
     if ( unlikely(xatpb->size < extent) )
         return -EILSEQ;