Re: [Qemu-devel] [PATCH 13/13] qdev-properties: Add pci-devaddr property

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 2012-06-10 at 17:54 +0300, Michael S. Tsirkin wrote:
> On Sun, Jun 10, 2012 at 08:41:03AM -0600, Alex Williamson wrote:
> > On Sun, 2012-06-10 at 17:03 +0300, Michael S. Tsirkin wrote:
> > > On Sun, Jun 10, 2012 at 07:41:51AM -0600, Alex Williamson wrote:
> > > > > > >>>> vfio_pci.c contains a nice function called "parse_hostaddr". You may
> > > > > > >>>> guess what it does. ;)
> > > > > > >>>
> > > > > > >>> Interesting. Why? This looks strange to me:
> > > > > > >>> I would expect the admin to bind a device to vfio
> > > > > > >>> the way it's now bound to a stub.
> > > > > > >>> The pass /dev/vfioXXX to qemu.
> > > > > > >>
> > > > > > >> That's the "libvirt way". We surely also want the "qemu command line
> > > > > > >> way" for which this kind of service is needed.
> > > > > > >>
> > > > > > >> Jan
> > > > > > >>
> > > > > > > 
> > > > > > > Yes, I imagine the qemu command line passing in /dev/vfioXXX,
> > > > > > > the libvirt way will pass in an fd for above. No?
> > > > > > 
> > > > > > As far as I understand the API, there is no device file per assigned
> > > > > > device.
> > > > > 
> > > > > Does it do pci_get_domain_bus_and_slot like kvm then?
> > > > > With all the warts like you have to remember to bind pci stub
> > > > > or you get two drivers for one device?
> > > > > If true that's unfortunate IMHO.
> > > 
> > > I hope the answer to the above is no?
> > 
> > No, it does a probe for devices.  We need the devaddr to compare against
> > dev_name of the device to figure out which device the user is attempting
> > to identify.
> > 
> > > > > > Also, this [domain:]bus:dev.fn format is more handy for the
> > > > > > command line.
> > > > > > 
> > > > > > Jan
> > > > > > 
> > > > > 
> > > > > Then users could add udev rules that will name vfio devices
> > > > > like this.  Another interesting option: /dev/vfio/eth0/vf1.
> > > > > That's better I think: no one really likes running lspci
> > > > > and guessing the address from there.
> > > > 
> > > > That's not at all how VFIO works.  /dev/vfio/# represents a group, which
> > > > may contain one or more devices.  Even if libvirt passes a file
> > > > descriptor for the group, qemu needs to know which device in the group
> > > > to add to the guest, so parsing a device address is still necessary.
> > > > Thanks,
> > > > 
> > > > Alex
> > > 
> > > That's very unusual, and unfortunate.  For example this means that I
> > > must update applications just because I move a card to another slot.
> > > UIO does not have this problem.
> > > The fact that it's broken in kvm ATM seems to have made people
> > > think it's okay, but it really is a bug. We didn't fix it
> > > because vfio was supposed to be the solution.
> > 
> > I don't know what you're talking about here.  Are you suggesting that
> > needing to specify -device pci-assign,host=3.0 changing to host=4.0 when
> > you move a card is broken?
> 
> Yes. Absolutely. Admin should be able to abstract it away without users
> knowing anything about it.

We don't have UUIDs on PCI devices, so who's to say that the device that
was in slot 3 is the same device that's now in slot 4 and the user
should still have access to it?  That sounds even more problematic.

> >  How does UIO avoid such a problem.
> 
> Normally you use a misc device that you can name with udev.
> 
> >  UIO-pci
> > requires the user to use pci-sysfs for resource access, so it surely
> > cares about the device address.
> 
> Only uio_pci_generic. Other uio devices let you drive the
> device.

If this is actually a problem, this is the first ever complaint I've
heard about it.  As above, I don't think we can assume the same access
when a device is moved.

> > > I do realize you want to represent a group of devices somehow but can't
> > > this be solved without breaking naming devices with udev? For example, the
> > > device could be a file as well. You would then use the fd to identify the
> > > device within the group. And in a somewhat common case of a single device
> > > within the group, you can even make opening the group optional.
> > > Don't know if this fix I suggest makes sense at all but it's a real
> > > problem all the same.
> > 
> > Unfortunately, exposing individual devices just confuses the ownership
> > model we require for groups.  It would provide the illusion of being
> > able to assign an individual device, without the reality of the
> > grouping.  Groups are owned either by _a_ user or by the kernel, they
> > can't be split across multiple users (at least not with any guarantees
> > of isolation).  The current interface makes this clear.  Thanks,
> > 
> > Alex
> 
> So do users pass in group=/dev/vfio/1,host=0:3.0 then?

No, vfio syntax is -device vfio-pci,host=0:3.0, just like pci-assign.
Qemu will figure out which group that device belongs to and "do the
right thing".  If we add support for libvirt passing a groupfd, it will
be mostly the same, just using scm_rights to get the groupfd instead of
opening it directly.  Thanks,

Alex