From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48361) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgr7X-0004uH-Ka for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cgr7U-000855-Jq for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:35 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34322) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cgr7U-00084v-Ep for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:32 -0500 From: "Daniel P. Berrange" Date: Thu, 23 Feb 2017 10:59:22 +0000 Message-Id: <20170223105922.22989-1-berrange@redhat.com> Subject: [Qemu-devel] [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Paolo Bonzini , Stefan Hajnoczi , Jitendra Kolhe , Michal Privoznik , "Daniel P. Berrange" 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 --- 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