On 08/22/13 18:10, Paolo Bonzini wrote:
> The thread from yesterday has died off (perhaps also because of
> my inappropriate answer to Michael, for which I apologize to him
> and everyone). I took some time to discuss the libvirt requirements
> further with Daniel Berrange and Eric Blake on IRC. If anyone is
> interested, I can give logs. This is a suggestion for how to
> proceed in both QEMU and libvirt.
The analysis is pretty overwhelming :)
I have read Anthony's response and I'm not trying to argue -- I'm just
spending a few thoughts on this and I'm willing to let them go to waste.
In general I think we should minimize the quirks the user (who edits the
libvirt XML) has to know about. Interactions between some XML elements,
without explicit inter-references (formulated as attributes, like
controller/index) are Bad (TM). So,
> == Builtin pvpanic ==
>
> QEMU will remove pvpanic from pc-1.5 in 1.6.1 and 1.5.4. This does not
> break migration.
>
>
> == Support in libvirt for current functionality ==
>
> libvirt will add a <panic-notifier/> element, and possibly a capability
> for it accessible via "virsh capabilities". There are two possibilities:
>
> 1) On QEMU 1.5.4/1.6.1 and newer (and on QEMU 1.6.0 with a machine type
> other than pc-1.5), <on_crash> will only work if the element is there.
> On QEMU 1.5.0->1.5.3, and on QEMU 1.6.0 with the pc-1.5 machine type,
> <on_crash> will be obeyed always, and may override e.g. reboot-on-panic
> if a guest driver exist.
I don't like this because there's some interplay between on_crash and
panic_notifier, which even depends on the qemu version.
>
> 2) On all versions, <on_crash> will only work if the element is there.
I like this, because, if on_crash doesn't work without panic_notifier
*at all*, then we can just drop panic_notifier, and make on_crash mean
(on_crash && panic_notifier) in the original sense.
IOW, drop "panic_notifier", and make "on_crash" work *always*.
>
>
> In turn, there are two ways to implement (2):
>
> 2a) libvirt will always add -global pvpanic.iobase=0 to neutralize
> the builtin pvpanic device if present. <panic-notifier/>
> will create the device with -device pvpanic,iobase=0x505
>
> Advantage: no changes to QEMU
>
> Disadvantage 1: writes to port 0 with QEMU 1.{5.0,5.1,5.2,5.3,6.0}
> and pc-1.5 machine type will write to a pvpanic device instead of
> the DMA controller. Probably harmless, and limited to some QEMU
> versions.
>
> Disadvantage 2: libvirt has knowledge of the pvpanic port number
Updating this paragraph with my above suggestion:
- (s/pvpanic.iobase/pvpanic.ioport/g)
- if "on_crash" is absent:
- for 1.{5.0,5.1,5.2,5.3,6.0}, add -global pvpanic.ioport=0
- for other versions, do nothing
- if "on_crash" is present:
- for 1.{5.0,5.1,5.2,5.3,6.0}, do nothing,
- for other versions, pass -device pvpanic
(knowledge of 0x505 is unneeded)
"advantage" and "disadvantage 1" remain, "disadvantage 2" is gone.
> 2b) QEMU will provide a way for libvirt to detect that no machine type
> has the builtin pvpanic. If some machine type may have the builtin
> pvpanic, and <panic-notifier/> is absent, libvirt will add
> "-global pvpanic.iobase=0" to neutralize it. Otherwise, libvirt
> will create the device normally.
>
> A possible way for libvirt to detect "good" machine types is a
> dummy property. This is a bit ugly in that the property would not
> affect the behavior of the device. The property would remain in
> the long term.
>
> Another possibility is for QEMU to rename the device, e.g. to
> isa-pvpanic. This is also somewhat gross, but not visible in the
> long term when the "pvpanic" name will be lost in history.
>
> Advantage 1: libvirt has no knowledge of the pvpanic port number
>
> Disadvantage 1: same as above
>
> Disadvantage 2: need a somewhat gross change in QEMU
>
>
> This method also provides an (also somewhat gross on the QEMU side)
> way to detect other changes in the pvpanic semantics. One example
> mentioned below, is making the panicked state temporary.
Too much work in qemu, in order to introduce ugliness, to hide older
ugliness.
> == Possible improvements to pvpanic ==
That's too complex / far out for me now, sorry :)
Thanks,
Laszlo
--
libvir-list mailing list
libvir-list@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/libvir-list