Re: [Qemu-devel] [PATCH V11 0/8] add pvpanic mmio support

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Dec 04, 2018 at 12:59:51PM +0000, Peter Maydell wrote:
> On Tue, 4 Dec 2018 at 12:47, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > After it had merged there were some changes and the question of turning
> > it into a PCI device was raised. Paolo was concerned that the guest OS
> > is in an unknown state (arbitrary locks held, data structures corrupt,
> > etc) when panic is fired, so simplicity of the I/O port was desirable:
> >
> >   https://lists.gnu.org/archive/html/qemu-devel/2013-08/msg03309.html
> >
> > Anthony countered that even a PCI device could simply do an outb() in
> > config space:
> >
> >   https://lists.gnu.org/archive/html/qemu-devel/2013-08/msg03325.html
> >
> > So it is not clear using a PCI device is in fact a problem in terms of
> > reliability at time of firing.
> 
> ...and if we'd done it that way in the first place for x86 then
> we wouldn't be having to do anything at all now for Arm.
> That suggests to me that we should do it that way now, and then we
> can avoid having to do a bunch of extra development work for the
> next architecture, or the next interesting Arm board model.

On s390 there's always a panic notifier mechanism as it is a
integral part of the architecture.

On PowerPC, pSeries guests have access to a panic notifier provided
by the firmware.

On x86, as well as pvpanic, there is also a paravirtualized
option defined by the HyperV extensions "hv_crash"

IIUC a PCI based solution would be usable on x86, s390, powerpc (pseries),
aarch64 (virt) and eventually riscv (virt). Of those, it is only
aarch64 and riscv that lack a panic notifier solution today.

I feel like we've already lost from the pov of a standardized solution,
but that doesn't mean we shouldn't still consider using PCI if it does
look like the best otion for arm/riscv.

Regards,
Daniel
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