Re: [Qemu-devel] [RESEND PATCH 2/2] hostmem-file: add an attribute 'align' to set its alignment

[Eduardo Habkost]

On Fri, May 26, 2017 at 07:24:26AM -0700, Dan Williams wrote:
> On Fri, May 26, 2017 at 12:16 AM, Haozhong Zhang
> <address@hidden> wrote:
> > On 05/26/17 07:05 +0000, Marc-André Lureau wrote:
> >> Hi
> >>
> >> On Fri, May 26, 2017 at 10:51 AM Haozhong Zhang <address@hidden>
> >> wrote:
> >>
> >> > On 05/26/17 06:39 +0000, Marc-André Lureau wrote:
> >> > > Hi
> >> > >
> >> > > On Fri, May 26, 2017 at 6:34 AM Haozhong Zhang <address@hidden
> >> > >
> >> > > 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