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

On 2014-06-17 16:23:58 Tue, Paul Mackerras wrote:

On Wed, Jun 11, 2014 at 02:18:21PM +0530, Mahesh J Salgaonkar wrote:

quoted

From: Mahesh Salgaonkar <redacted>

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.

Yes agree. I have patch (below) ready for the same, will test/verify and send it
out soon.

Thanks,
-Mahesh.

-------------
Deliver machine check with MSR(RI=0) to guest as MCE

From: Mahesh Salgaonkar <redacted>


---
 arch/powerpc/kvm/book3s_hv_rmhandlers.S |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index 868347e..c9c56ee 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -2257,7 +2257,6 @@ 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
 	/*

@@ -2270,13 +2269,18 @@ 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