Re: [Qemu-devel] [PATCH] `qdev_free` when unplug a pci device

[Ryan Harper <ryanh@us.ibm.com>]

* Wen Congyang <wency@cn.fujitsu.com> [2011-03-09 01:21]:
> At 03/09/2011 02:12 PM, Ryan Harper Write:
> > * Wen Congyang <wency@cn.fujitsu.com> [2011-03-08 23:09]:
> >> At 03/09/2011 12:08 PM, Ryan Harper Write:
> >>> * Wen Congyang <wency@cn.fujitsu.com> [2011-02-27 20:56]:
> >>>> Hi Markus Armbruster
> >>>>
> >>>> At 02/23/2011 04:30 PM, Markus Armbruster Write:
> >>>>> Isaku Yamahata <yamahata@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
ryanh@us.ibm.com