From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:56940) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1grxu4-0007JB-B8 for qemu-devel@nongnu.org; Thu, 07 Feb 2019 23:36:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1grxu2-00020Y-FE for qemu-devel@nongnu.org; Thu, 07 Feb 2019 23:36:40 -0500 Received: from mga03.intel.com ([134.134.136.65]:57762) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1grxu0-0001wv-MT for qemu-devel@nongnu.org; Thu, 07 Feb 2019 23:36:38 -0500 Date: Fri, 8 Feb 2019 21:07:14 +0800 From: Yi Zhang Message-ID: <20190208130713.GA63124@tiger-server> References: <42dd91019c7aa480cb0d26565fefcb1238c37dc8.1549555521.git.yi.z.zhang@linux.intel.com> <20190207215556-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190207215556-mutt-send-email-mst@kernel.org> Subject: Re: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: xiaoguangrong.eric@gmail.com, stefanha@redhat.com, pbonzini@redhat.com, pagupta@redhat.com, yu.c.zhang@linux.intel.com, richardw.yang@linux.intel.com, ehabkost@redhat.com, qemu-devel@nongnu.org, imammedo@redhat.com, dan.j.williams@intel.com On 2019-02-07 at 22:00:46 -0500, Michael S. Tsirkin wrote: > On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote: > > From: Zhang Yi > > > > Signed-off-by: Zhang Yi > > --- > > docs/nvdimm.txt | 25 ++++++++++++++++++++++--- > > qemu-options.hx | 5 +++++ > > 2 files changed, 27 insertions(+), 3 deletions(-) > > > > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > > index 5f158a6..a168429 100644 > > --- a/docs/nvdimm.txt > > +++ b/docs/nvdimm.txt > > @@ -143,9 +143,28 @@ Guest Data Persistence > > ---------------------- > > > > Though QEMU supports multiple types of vNVDIMM backends on Linux, > > -currently the only one that can guarantee the guest write persistence > > -is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to > > -which all guest access do not involve any host-side kernel cache. > > +the only backend that can guarantee the guest write persistence is: > > + > > +A. DAX device (e.g., /dev/dax0.0, ) or > > +B. DAX file(mounted with dax option) > > + > > +both are use real NVDIMM device as backend, which supporting direct > > +access for files(no page cache). > > So I would just drop above two lines. > > How does one know that there's support for direct access? > it's just a question of kernel right? > > And I don't think it matter wrt "real NVDIMM" - e.g. it > could be vNVDIMM in a nested virt scenario. > It will still survive host (i.e. L1 guest) crash. > > Besides, the above two lines aren't grammatical. Ah.. Reasonable, will drop these two lines. Thanks for your kindly review. Michael. > > > + > > +When using B (A file supporting direct mapping of persistent memory) > > +as a backend, write persistence is guaranteed if the host kernel has > > +support for the MAP_SYNC flag in the mmap system call (available > > +since Linux 4.15 and on certain distro kernels) and additionally > > +both 'pmem' and 'share' flags are set to 'on' on the backend. > > + > > +If these conditions are not satisfied i.e. if either 'pmem' or 'share' > > +are not set, if the backend file does not support DAX or if MAP_SYNC > > +is not supported by the host kernel, write persistence is not > > +guaranteed after a system crash. For compatibility reasons, these > > +conditions are silently ignored if not satisfied. Currently, no way > > +is provided to test for them. > > +For more details, please reference mmap(2) man page: > > +http://man7.org/linux/man-pages/man2/mmap.2.html. > > > > When using other types of backends, it's suggested to set 'unarmed' > > option of '-device nvdimm' to 'on', which sets the unarmed flag of the > > diff --git a/qemu-options.hx b/qemu-options.hx > > index 08f8516..ef1da8f 100644 > > --- a/qemu-options.hx > > +++ b/qemu-options.hx > > @@ -4002,6 +4002,11 @@ using the SNIA NVM programming model (e.g. Intel NVDIMM). > > If @option{pmem} is set to 'on', QEMU will take necessary operations to > > guarantee the persistence of its own writes to @option{mem-path} > > (e.g. in vNVDIMM label emulation and live migration). > > +Also, we will map the backend-file with MAP_SYNC flag, which ensures the > > +file metadata is in sync for @option{mem-path} in case of host crash > > +or a power failure. MAP_SYNC requires support from both the host kernel > > +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted > > +with DAX option. > > > > @item -object memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave} > > > > -- > > 2.7.4