On Fri, May 26, 2017 at 12:16 AM, Haozhong Zhang <haozhong.zh...@intel.com> wrote: > On 05/26/17 07:05 +0000, Marc-André Lureau wrote: >> Hi >> >> On Fri, May 26, 2017 at 10:51 AM Haozhong Zhang <haozhong.zh...@intel.com> >> wrote: >> >> > On 05/26/17 06:39 +0000, Marc-André Lureau wrote: >> > > Hi >> > > >> > > On Fri, May 26, 2017 at 6:34 AM Haozhong Zhang <haozhong.zh...@intel.com >> > > >> > > wrote: >> > > >> > > > file_ram_alloc() currently maps the backend file via mmap to a virtual >> > > > address aligned to the value returned by qemu_fd_getpagesize(). When a >> > > > DAX device (e.g. /dev/dax0.0) is used as the backend file, its kernel >> > > > mmap implementation may require an alignment larger than what >> > > > qemu_fd_get_pagesize() returns (e.g. 2MB vs. 4KB), and mmap may fail. >> > > > >> > > >> > > How is the accepted value queried? Any chance to configure it >> > > automatically? >> > >> > Take /dev/dax0.0 for example. The value can be read from >> > /sys/class/dax/dax0.0/device/dax_region/align. >> > >> >> Should this work be left to management layer, or could qemu figure it out >> by itself (using udev?) >> > > For DAX device only, QEMU can figure out the proper alignment by > itself. However, I'm not sure whether there are other non-DAX cases > requiring non-default alignment, so I think it's better to just add an > interface (i.e. align attribute) in QEMU and let other management > tools (e.g. libvirt?) fill a proper value.
I can't imagine any cases where you would want to specify an alignment. If it's regular file mmap any alignment is fine, and if it's device-dax only the configured alignment of the device instance is allowed. So, I don't think this should be a configurable option, just read it from the device instance and you're done.
