Re: [Xen-devel] [PATCH 0/3][RFC] PV Passthrough PCI Device Hotplug Support

[Yosuke Iwamatsu <y-iwamatsu@xxxxxxxxxxxxx>]

Keir Fraser wrote:
> On 21/2/08 13:25, "Yosuke Iwamatsu" <y-iwamatsu@xxxxxxxxxxxxx> wrote:
> > dev-#, vdev-# and state-# are all backend nodes.
> > Xend writes physical names on dev-# to let pciback know which device
> > should be exported (This is the original behaviour).
> > Then pciback publishes the corresponding virtul name on vdev-#.
> > At the time of detachment, pcifront scans backend nodes and finds
> > which device should be removed by seeing vdev-#.
> > pcifront doesn't need to know the physical name indeed.
> This seems a bit different from how initial probe happens (by reading the
> root-%d xenstore nodes). Could we not just watch those in pcifront and
> re-parse them for changes?
>
> Your state machine was also quite complicated, perhaps primarily so that
> pcifront could handshake when devices are removed. Is this handshake
> necessary? (e.g., in particular, is there any equivalent notion for real PCI
> hot-unplug?). If not, we can get equivalent hotplug functionality between
> HVM and PV without needing a complicated xend->frontend->backend->xend
> handshaking ring:
>  * xend updates dev-% nodes
>  * pciback sees this and updates root-%d nodes
>  * pcifront sees this and re-parses root-%d nodes
When hot add, this method (xend->pciback->pcifront) works.
And I did that way in my patch.

When hot remove, however, I'm afraid it's not safe.
xend->pciback->pcifront means that we first disable io/mmio port access,
destroy backend config space emulation and then notify the guest OS of
the removal. The guest OS would see something like a virtual
surprise-style removal, that is, a device suddenly disappears
while it is in use.

I confess that I have never done a real PCI hot-unplug myself, but
several specs (acpi spec 3.0b 6.3, pcihp spec 1.1) say that a
recommended hot removal sequence is like:
  1. the user notifys the OS of the desire to remove a slot
  2. the OS logicaly removes the device (unload driver, poweroff, etc)
     and report the user it's ready
  3. the user removes the slot
So the reason I added vdev-# and I chose xend->frontend->backend->xend
cicle is the same; let the guest OS cleanly shutdown the device first
and then actually remove it. I think it's quite essential.

Thanks,
  Yosuke

>  -- Keir

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