On Fri, May 26, 2017 at 07:24:26AM -0700, Dan Williams wrote: > 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.
Agreed. BTW, there's no generic interface to ask the kernel what's the required mmap() alignment for a file? -- Eduardo
