From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefano Stabellini Subject: Re: [RFC Design Doc] Add vNVDIMM support for Xen Date: Thu, 4 Feb 2016 12:24:20 +0000 Message-ID: References: <20160201054414.GA25211@hz-desktop.sh.intel.com> <20160203070052.GA4248@hz-desktop.sh.intel.com> <56B219D4.9030507@citrix.com> <20160204025526.GA3504@hz-desktop.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160204025526.GA3504@hz-desktop.sh.intel.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Haozhong Zhang Cc: Juergen Gross , Kevin Tian , Keir Fraser , Ian Campbell , Stefano Stabellini , George Dunlap , Andrew Cooper , Ian Jackson , George Dunlap , Xiao Guangrong , xen-devel@lists.xen.org, Jan Beulich , Jun Nakajima , Wei Liu List-Id: xen-devel@lists.xenproject.org On Thu, 4 Feb 2016, Haozhong Zhang wrote: > On 02/03/16 15:22, Stefano Stabellini wrote: > > On Wed, 3 Feb 2016, George Dunlap wrote: > > > On 03/02/16 12:02, Stefano Stabellini wrote: > > > > On Wed, 3 Feb 2016, Haozhong Zhang wrote: > > > >> Or, we can make a file system on /dev/pmem0, create files on it, set > > > >> the owner of those files to xen-qemuuser-domid$domid, and then pass > > > >> those files to QEMU. In this way, non-root QEMU should be able to > > > >> mmap those files. > > > > > > > > Maybe that would work. Worth adding it to the design, I would like to > > > > read more details on it. > > > > > > > > Also note that QEMU initially runs as root but drops privileges to > > > > xen-qemuuser-domid$domid before the guest is started. Initially QEMU > > > > *could* mmap /dev/pmem0 while is still running as root, but then it > > > > wouldn't work for any devices that need to be mmap'ed at run time > > > > (hotplug scenario). > > > > > > This is basically the same problem we have for a bunch of other things, > > > right? Having xl open a file and then pass it via qmp to qemu should > > > work in theory, right? > > > > Is there one /dev/pmem? per assignable region? > > Yes. > > BTW, I'm wondering whether and how non-root qemu works with xl disk > configuration that is going to access a host block device, e.g. > disk = [ '/dev/sdb,,hda' ] > If that works with non-root qemu, I may take the similar solution for > pmem. Today the user is required to give the correct ownership and access mode to the block device, so that non-root QEMU can open it. However in the case of PCI passthrough, QEMU needs to mmap /dev/mem, as a consequence the feature doesn't work at all with non-root QEMU (http://marc.info/?l=xen-devel&m=145261763600528). If there is one /dev/pmem device per assignable region, then it would be conceivable to change its ownership so that non-root QEMU can open it. Or, better, the file descriptor could be passed by the toolstack via qmp.