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

[Dan Williams <dan.j.williams@intel.com>]

On Fri, May 26, 2017 at 12:16 AM, Haozhong Zhang
<haozhong.zhang@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.zhang@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.zhang@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.