[Qemu-devel] [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc

["Daniel P. Berrange" <berrange@redhat.com>]

When using a memory-backend object with prealloc turned on, QEMU
will memset() the first byte in every memory page to zero. While
this might have been acceptable for memory backends associated
with RAM, this corrupts application data for NVDIMMs.

Instead of setting every page to zero, read the current byte
value and then just write that same value back, so we are not
corrupting the original data.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---

I'm unclear if this is actually still safe in practice ? Is the
compiler permitted to optimize away the read+write since it doesn't
change the memory value. I'd hope not, but I've been surprised
before...

IMHO this is another factor in favour of requesting an API from
the kernel to provide the prealloc behaviour we want.

 util/oslib-posix.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 35012b9..8f5b656 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -355,7 +355,8 @@ void os_mem_prealloc(int fd, char *area, size_t memory, Error **errp)
 
         /* MAP_POPULATE silently ignores failures */
         for (i = 0; i < numpages; i++) {
-            memset(area + (hpagesize * i), 0, 1);
+            char val = *(area + (hpagesize * i));
+            memset(area + (hpagesize * i), 0, val);
         }
     }
 
-- 
2.9.3