Re: [Qemu-devel] [PATCH] vl: allow "cont" from panicked state

[Laszlo Ersek <lersek@redhat.com>]

On 08/22/13 11:19, Paolo Bonzini wrote:
> Il 22/08/2013 10:38, Laszlo Ersek ha scritto:
>>>> To support 1.5, libvirt should simply be ready to react to unanticipated
>>>> GUEST_PANICKED events.  reboot-on-panic will simply be broken for 1.5
>>>> and Linux 3.10+ guests. :(
>> I'm probably misunderstanding the discussion, but it might be possible
>> to disable pvpanic even in 1.5 from the host side, with the following hack:
>>
>>   -global pvpanic.ioport=0
>>
>> In qemu, this will either configure a working pvpanic device on ioport
>> 0, or the pvpanic device will be genuinely broken. At least it doesn't
>> (obviously) break other stuff (in v1.5.2):
>>
>> (qemu) info mtree
>> I/O
>> 0000000000000000-000000000000ffff (prio 0, RW): io
>>   0000000000000000-0000000000000000 (prio 0, RW): pvpanic
>>   0000000000000000-0000000000000007 (prio 0, RW): dma-chan
> 
> No, you're not misunderstanding the discussion.
> 
> Depending on the priorities of the pvpanic and legacy-DMA regions, it
> would break DMA channel 0. 

<academic>

I think before priority comes into the picture, the access size would
matter first, no?

(I think I'm recalling this from the 0xCF9 reset control register, which
falls into the [0xCF8..0xCFA] range.)

Unless ioport 0 is accessed with width 1 for dma-chan purposes, I think
such an access would be unique to pvpanic, and always dispatched to pvpanic.

> Channel 0 is (was) used for DRAM refresh, so
> it should not have any visible effect.  However, it may not be entirely
> disabling pvpanic, just making it mostly invisible.

That's good enough for the guest to reach kexec :)

</academic>

Laszlo