KVM: PPC: Book3S HV: Deliver machine check with MSR(RI=0) to guest as MCE

For the machine check interrupt that happens while we are in the guest,
kvm layer attempts the recovery, and then delivers the machine check interrupt
directly to the guest if recovery fails. On successful recovery we go back to
normal functioning of the guest. But there can be cases where a machine check
interrupt can happen with MSR(RI=0) while we are in the guest. This means
MC interrupt is unrecoverable and we have to deliver a machine check to the
guest since the machine check interrupt might have trashed valid values in
SRR0/1. The current implementation do not handle this case, causing guest
to crash with Bad kernel stack pointer instead of machine check oops message.

[26281.490060] Bad kernel stack pointer 3fff9ccce5b0 at c00000000000490c
[26281.490434] Oops: Bad kernel stack pointer, sig: 6 [#1]
[26281.490472] SMP NR_CPUS=2048 NUMA pSeries

This patch fixes this issue by checking MSR(RI=0) in KVM layer and forwarding
unrecoverable interrupt to guest which then panics with proper machine check
Oops message.
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -2377,7 +2377,6 @@ machine_check_realmode:
 	mr	r3, r9		/* get vcpu pointer */
 	bl	kvmppc_realmode_machine_check
 	nop
-	cmpdi	r3, 0		/* Did we handle MCE ? */
 	ld	r9, HSTATE_KVM_VCPU(r13)
 	li	r12, BOOK3S_INTERRUPT_MACHINE_CHECK
 	/*
@@ -2390,13 +2389,18 @@ machine_check_realmode:
 	 * The old code used to return to host for unhandled errors which
 	 * was causing guest to hang with soft lockups inside guest and
 	 * makes it difficult to recover guest instance.
+	 *
+	 * if we receive machine check with MSR(RI=0) then deliver it to
+	 * guest as machine check causing guest to crash.
 	 */
-	ld	r10, VCPU_PC(r9)
 	ld	r11, VCPU_MSR(r9)
+	andi.	r10, r11, MSR_RI	/* check for unrecoverable exception */
+	beq	1f			/* Deliver a machine check to guest */
+	ld	r10, VCPU_PC(r9)
+	cmpdi	r3, 0		/* Did we handle MCE ? */
 	bne	2f	/* Continue guest execution. */
 	/* If not, deliver a machine check.  SRR0/1 are already set */
-	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
-	ld	r11, VCPU_MSR(r9)
+1:	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
 	bl	kvmppc_msr_interrupt
 2:	b	fast_interrupt_c_return