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

[Haozhong Zhang]

On 05/26/17 15:55 -0300, Eduardo Habkost wrote:
> 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.
>

Ok, I'll drop this attribute and let QEMU figure out the alignment.

Thanks,
Haozhong

> BTW, there's no generic interface to ask the kernel what's the
> required mmap() alignment for a file?
> 
> -- 
> Eduardo
>