From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34965) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ekA7U-0004DI-5M for qemu-devel@nongnu.org; Fri, 09 Feb 2018 09:57:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ekA7P-0006Oa-AD for qemu-devel@nongnu.org; Fri, 09 Feb 2018 09:57:44 -0500 Received: from mga09.intel.com ([134.134.136.24]:18302) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ekA7P-0006Nt-06 for qemu-devel@nongnu.org; Fri, 09 Feb 2018 09:57:39 -0500 Date: Fri, 9 Feb 2018 22:57:26 +0800 From: Haozhong Zhang Message-ID: <20180209145726.55ushghorccxne47@hz-desktop> References: <20180207073331.14158-1-haozhong.zhang@intel.com> <20180207073331.14158-5-haozhong.zhang@intel.com> <20180209142740.GA3390@stefanha-x1.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180209142740.GA3390@stefanha-x1.localdomain> Subject: Re: [Qemu-devel] [PATCH v2 4/8] mem/nvdimm: ensure write persistence to PMEM in label emulation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Eduardo Habkost , Igor Mammedov , Paolo Bonzini , mst@redhat.com, Xiao Guangrong , Juan Quintela , dgilbert@redhat.com, Dan Williams On 02/09/18 14:27 +0000, Stefan Hajnoczi wrote: > On Wed, Feb 07, 2018 at 03:33:27PM +0800, Haozhong Zhang wrote: > > @@ -156,11 +157,17 @@ static void nvdimm_write_label_data(NVDIMMDevice *nvdimm, const void *buf, > > { > > MemoryRegion *mr; > > PCDIMMDevice *dimm = PC_DIMM(nvdimm); > > + bool is_pmem = object_property_get_bool(OBJECT(dimm->hostmem), > > + "pmem", NULL); > > uint64_t backend_offset; > > > > nvdimm_validate_rw_label_data(nvdimm, size, offset); > > > > - memcpy(nvdimm->label_data + offset, buf, size); > > + if (!is_pmem) { > > + memcpy(nvdimm->label_data + offset, buf, size); > > + } else { > > + pmem_memcpy_persist(nvdimm->label_data + offset, buf, size); > > + } > > Is this enough to prevent label corruption in case of power failure? > > pmem_memcpy_persist() is not atomic. Power failure can result in a mix > of the old and new label data. > > If we want this operation to be 100% safe there needs to be some kind of > update protocol that makes the change atomic, like a Label A and Label B > area with a single Label Index field that can be updated atomically to > point to the active Label A/B area. All this patch series is to guarantee: if the guest is still alive and running, all its previous writes to pmem, which were performed by QEMU, will be still persistent on pmem. If a power failure happens before QEMU returns to the guest, e.g., in the middle of above pmem_memcpy_persist(), yes, the guest label data may be in an inconsistent state, but the guest also has no chance to progress. And, that is what could happen in the non-virtualization environment as well, and it's the responsibility of the (guest) SW to defend such failures, e.g., by the protocol you mentioned. Haozhong