[Qemu-devel] [PATCH] ppc: Disable huge page support if it is not available for main RAM

[Thomas Huth <thuth@redhat.com>]

On powerpc, we must only signal huge page support to the guest if
all memory areas are capable of supporting huge pages. The commit
2d103aae8765 ("fix hugepage support when using memory-backend-file")
already fixed the case when the user specified the mem-path property
for NUMA memory nodes instead of using the global "-mem-path" option.
However, there is one more case where it currently can go wrong.
When specifying additional memory DIMMs without using NUMA, e.g.

 qemu-system-ppc64 -enable-kvm ... -m 1G,slots=2,maxmem=2G \
    -device pc-dimm,id=dimm-mem1,memdev=mem1 -object \
    memory-backend-file,policy=default,mem-path=/...,size=1G,id=mem1

the code in getrampagesize() currently assumes that huge pages
are possible since they are enabled for the mem1 object. But
since the main RAM is not backed by a huge page filesystem,
the guest Linux kernel then crashes very quickly after being
started. So in case the we've got "normal" memory without NUMA
and without the global "-mem-path" option, we must not announce
huge pages to the guest. Since this is likely a mis-configuration
by the user, also spill out a message in this case.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 target-ppc/kvm.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index e14da60..884d564 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -27,6 +27,7 @@
 #include "qemu/timer.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/kvm.h"
+#include "sysemu/numa.h"
 #include "kvm_ppc.h"
 #include "sysemu/cpus.h"
 #include "sysemu/device_tree.h"
@@ -388,7 +389,21 @@ static long getrampagesize(void)
 
     object_child_foreach(memdev_root, find_max_supported_pagesize, &hpsize);
 
-    return (hpsize == LONG_MAX) ? getpagesize() : hpsize;
+    if (hpsize == LONG_MAX) {
+        return getpagesize();
+    }
+
+    if (nb_numa_nodes == 0 && hpsize > getpagesize()) {
+        /* No NUMA nodes and normal RAM without -mem-path ==> no huge pages! */
+        static bool warned;
+        if (!warned) {
+            error_report("Huge page support disabled (n/a for main memory).");
+            warned = true;
+        }
+        return getpagesize();
+    }
+
+    return hpsize;
 }
 
 static bool kvm_valid_page_size(uint32_t flags, long rampgsize, uint32_t shift)
-- 
1.8.3.1