From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:32997) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgs9S-00080r-VE for qemu-devel@nongnu.org; Thu, 23 Feb 2017 07:05:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cgs9S-0007zu-1c for qemu-devel@nongnu.org; Thu, 23 Feb 2017 07:05:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45508) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cgs9R-0007zG-Jq for qemu-devel@nongnu.org; Thu, 23 Feb 2017 07:05:37 -0500 References: <20170223105922.22989-1-berrange@redhat.com> From: Michal Privoznik Message-ID: <57a2a646-cce2-2b65-bce5-793ffaa1ec9b@redhat.com> Date: Thu, 23 Feb 2017 13:05:33 +0100 MIME-Version: 1.0 In-Reply-To: <20170223105922.22989-1-berrange@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [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: "Daniel P. Berrange" , qemu-devel@nongnu.org Cc: Paolo Bonzini , Stefan Hajnoczi , Jitendra Kolhe On 02/23/2017 11:59 AM, Daniel P. Berrange wrote: > 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); I think you wanted: memset(area + (hpagesize * i), val, 1); because what you are suggesting will overwrite even more than the first byte with zeroes. Michal