[PATCH 4/4] powerpc/book3s: Fix guest MC delivery mechanism to avoid soft lockups in guest.

[Paul Mackerras]

On Wed, Jun 11, 2014 at 02:18:21PM +0530, Mahesh J Salgaonkar wrote:
> From: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
> 
> Currently we forward MCEs to guest which have been recovered by guest.
> And for unhandled errors we do not deliver the MCE to guest. It looks like
> with no support of FWNMI in qemu, guest just panics whenever we deliver the
> recovered MCEs to guest. Also, the existig code used to return to host for
> unhandled errors which was casuing guest to hang with soft lockups inside
> guest and makes it difficult to recover guest instance.
> 
> This patch now forwards all fatal MCEs to guest causing guest to crash/panic.
> And, for recovered errors we just go back to normal functioning of guest
> instead of returning to host.

... having corrupted possibly live values that the guest had in SRR0/1.

Ideally the guest should have cleared MSR[RI] before putting values in
SRR0/1, so perhaps you could check that and return to the guest
without giving it a machine check if MSR[RI] is set.  But if MSR[RI]
is clear, the guest is unfixably corrupted because the machine check
overwrote SRR0/1, and the only thing we can do, in the absence of
FWNMI support, is give the guest a machine check interrupt and let it
crash.

Paul.