Re: [Qemu-devel] Querying the size of devdax devices from userspace

[Dan Williams]

On Sun, Feb 3, 2019 at 11:56 PM Stefan Hajnoczi <address@hidden> wrote:
>
> How can userspace applications query the size of devdax character
> devices?
>
> stat(1) doesn't know how large the device is:
>
>   # stat /dev/dax0.0
>     File: /dev/dax0.0
>     Size: 0             Blocks: 0          IO Block: 4096   character special 
> file
>   Device: 6h/6d Inode: 56764       Links: 1     Device type: fa,d
>
> ndctl(1) finds out by digging in /sys:
>
>   # ndctl list
>   [
>     {
>       "dev":"namespace0.0",
>       "mode":"devdax",
>       "map":"dev",
>       "size":2111832064,
>       "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
>       "chardev":"dax0.0"
>     }
>   ]
>
> I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
> there a mapping from devdax major/minor number to a /sys path?
>
> The use case I have in mind is that QEMU currently takes the size as a
> command-line parameter.  The failure mode is ugly when users get this
> value wrong: the guest gets a softlockup and there is no error message
> on the host side.
>
> I'd like QEMU to detect the size or at least reject size values that are
> too large.  In order to do that userspace needs a convenient way of
> querying the size.  Any ideas?

You're looking for /sys/dev/char. That will wake you from major:minor
to sysfs. Perhaps borrow fio's implementation for the same:

   http://git.kernel.dk/cgit/fio/tree/engines/dev-dax.c#n258