[libvirt] RFC: Enable unprivileged SG_IO

[Osier Yang <jyang redhat com>]

Hi,

http://lwn.net/Articles/524720/ introduces new sysfs knob
(unpriv_sgio) for SCSI device to allow the unprivileged SG_IO.

I don't have solid thought on how the libvirt interface should
be yet. It shouldn't be a XML entry of disk device, as the device
can be shared by multiple guests, and the configuration should
be kept same for all of them, having a XML entry for it will
make things a mess.

What Paolo suggested is to add an entry in qemu.conf, just
like "cgroup_device_acl":

sgio_device_acl = [ "/dev/sda" ]

When libvirtd starting, set the sysfs knob "unpriv_sgio" of
the devices listed to 1, and 0 when libvirtd exists.

I don't quite agree with this approach, as entries in qemu.conf
generally should be configuration for the whole qemu driver,
however, the SG_IO setting is at the device layer, or not
higher than guest layer.

What I'm thinking about is to have a public API to tune the
knob independantly with domain/driver, that means it's up to
management apps to manage the knob's value, setting it to 1
before domain(s) starting, and 0 when no domain is using it.

Any thoughts?

Regards,
Osier