From 9dc44aa5829eb3131a01378a738dee28a382bbc1 Mon Sep 17 00:00:00 2001 From: "Daniel P. Berrange" Date: Fri, 3 Mar 2017 11:32:55 +0000 Subject: [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc 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. Directly write the value instead of memset()ing it, since there's no benefit to memset for a single byte write. Signed-off-by: Daniel P. Berrange Reviewed-by: Andrea Arcangeli Message-id: 20170303113255.28262-1-berrange@redhat.com Signed-off-by: Stefan Hajnoczi --- util/oslib-posix.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/util/oslib-posix.c b/util/oslib-posix.c index 956f66ab4a..94d81b9ec0 100644 --- a/util/oslib-posix.c +++ b/util/oslib-posix.c @@ -361,7 +361,19 @@ static void *do_touch_pages(void *arg) memset_thread_failed = true; } else { for (i = 0; i < numpages; i++) { - memset(addr, 0, 1); + /* + * Read & write back the same value, so we don't + * corrupt existing user/app data that might be + * stored. + * + * 'volatile' to stop compiler optimizing this away + * to a no-op + * + * TODO: get a better solution from kernel so we + * don't need to write at all so we don't cause + * wear on the storage backing the region... + */ + *(volatile char *)addr = *addr; addr += hpagesize; } }