[libvirt] pvpanic plans?
Laszlo Ersek
lersek at redhat.com
Thu Aug 22 19:41:40 UTC 2013
On 08/22/13 21:19, Paolo Bonzini wrote:
> Il 22/08/2013 19:15, Laszlo Ersek ha scritto:
>>> 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*.
>
> No, we cannot because of backwards compatibility. VMs could have no
> on_crash element (which means <on_crash>destroy</on_crash>) and yet the
> guest admin could expect them to reboot on panic.
Ah. I thought "no on_crash" meant <on_crash>ignore</on_crash>, or
something like that -- if on_crash was absent, the guest wouldn't see a
working pvpanic device in ACPI, and wouldn't trigger the event in qemu.
>>> 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.
>
> Is it too much work? s/"pvpanic"/"isa-pvpanic"?
... I probably skipped the rename option because you called it gross
(and maybe because I (erroneously?) recall Michael's opposition). I
think I meant the dummy property under "too much work" (it may not be,
in retrospect, but properties always imply compat stuff for me, and
*that* is scary).
Laszlo
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