* Wen Congyang <we...@cn.fujitsu.com> [2011-03-09 01:21]: > At 03/09/2011 02:12 PM, Ryan Harper Write: > > * Wen Congyang <we...@cn.fujitsu.com> [2011-03-08 23:09]: > >> At 03/09/2011 12:08 PM, Ryan Harper Write: > >>> * Wen Congyang <we...@cn.fujitsu.com> [2011-02-27 20:56]: > >>>> Hi Markus Armbruster > >>>> > >>>> At 02/23/2011 04:30 PM, Markus Armbruster Write: > >>>>> Isaku Yamahata <yamah...@valinux.co.jp> writes: > >>>>> > >>>> > >>>> <snip> > >>>> > >>>>> > >>>>> I don't think this patch is correct. Let me explain. > >>>>> > >>>>> Device hot unplug is *not* guaranteed to succeed. > >>>>> > >>>>> For some buses, such as USB, it always succeeds immediately, i.e. when > >>>>> the device_del monitor command finishes, the device is gone. Live is > >>>>> good. > >>>>> > >>>>> But for PCI, device_del merely initiates the ACPI unplug rain dance. It > >>>>> doesn't wait for the dance to complete. Why? The dance can take an > >>>>> unpredictable amount of time, including forever. > >>>>> > >>>>> Problem: Subsequent device_add can fail if it reuses the qdev ID or PCI > >>>>> slot, and the unplug has not yet completed (race condition), or it > >>>>> failed. Yes, Virginia, PCI hotplug *can* fail. > >>>>> > >>>>> When unplug succeeds, the qdev is automatically destroyed. > >>>>> pciej_write() does that for PIIX4. Looks like pcie_cap_slot_event() > >>>>> does it for PCIE. > >>>> > >>>> I got a similar problem. When I unplug a pci device by hand, it works > >>>> as expected, and I can hotplug it again. But when I use a srcipt to > >>>> do the same thing, sometimes it failed. I think I may find another bug. > >>>> > >>>> Steps to reproduce this bug: > >>>> 1. cat ./test-e1000.sh # RHEL6RC is domain name > >>>> #! /bin/bash > >>>> > >>>> while true; do > >>>> virsh attach-interface RHEL6RC network default --mac > >>>> 52:54:00:1f:db:c7 --model e1000 > >>>> if [[ $? -ne 0 ]]; then > >>>> break > >>>> fi > >>>> virsh detach-interface RHEL6RC network --mac 52:54:00:1f:db:c7 > >>>> if [[ $? -ne 0 ]]; then > >>>> break > >>>> fi > >>>> sleep 5 > >>> > >>> How do you know that the guest has responded at this point before you > >>> attempt to attach again at the top of the loop. Any attach/detach > >>> requires the guest to respond to the request and it may not respond at > >>> all. > >> > >> When I attach/detach interface by hand, it works fine: I can see the new > >> interface > >> when I attach it, and it disapears when I detached it. > > > > The point is that since the attach and detach require guest > > participation, this interface isn't reliable. You have a sleep 5 in > > your loop, hoping to wait long enough for the guest to respond, but > > after a number of iterations in your loop it fails, you can bump the > > sleep to to 3600 seconds and the guest *still* might not respond... > > We use sci interrupt to tell the guest that a device has been > attached/detached. > But the sci interrupt is *lost* in qemu, so the guest does not know a device > has > been attached/detached, and does not respond it. > > If the sci interrupt is not lost, the guest can respond it.
*can* is the important word. Even if the interrupt isn;t lost, you have no way to guarantee that the guest will respond at all. That's not to say there isn't a bug around the lost interrupt; but rather a more general point about hotplug's current architecture. > > > > > -- Ryan Harper Software Engineer; Linux Technology Center IBM Corp., Austin, Tx ry...@us.ibm.com
