Subject: [PATCH v2] KVM: PPC: Exit guest upon fatal machine check exception

From: Aravinda Prasad <[email-masked]>
Date: Wed, 16 Dec 2015 11:26:12 +0530

This patch modifies KVM to cause a guest exit with
KVM_EXIT_NMI instead of immediately delivering a 0x200
interrupt to guest upon machine check exception in
guest address. Exiting the guest enables QEMU to build
error log and deliver machine check exception to guest
OS (either via guest OS registered machine check
handler or via 0x200 guest OS interrupt vector).

This approach simplifies the delivering of machine
check exception to guest OS compared to the earlier approach
of KVM directly invoking 0x200 guest interrupt vector.
In the earlier approach QEMU patched the 0x200 interrupt
vector during boot. The patched code at 0x200 issued a
private hcall to pass the control to QEMU to build the
error log.

This design/approach is based on the feedback for the
QEMU patches to handle machine check exception. Details
of earlier approach of handling machine check exception
in QEMU and related discussions can be found at:

https://lists.nongnu.org/archive/html/qemu-devel/2014-11/msg00813.html

This patch also introduces a new KVM capability to
control how KVM behaves on machine check exception.
Without this capability, KVM redirects machine check
exceptions to guest's 0x200 vector if the address in
error belongs to guest. With this capability KVM
causes a guest exit with NMI exit reason.

This is required to avoid problems if a new kernel/KVM
is used with an old QEMU for guests that don't issue
"ibm,nmi-register". As old QEMU does not understand the
NMI exit type, it treats it as a fatal error. However,
the guest could have handled the machine check error
if the exception was delivered to guest's 0x200 interrupt
vector instead of NMI exit in case of old QEMU.

Change Log v2:
  - Added KVM capability

Signed-off-by: Aravinda Prasad <[email-masked]>
---
 arch/powerpc/include/asm/kvm_host.h     |    1 +
 arch/powerpc/kernel/asm-offsets.c       |    1 +
 arch/powerpc/kvm/book3s_hv.c            |   12 +++-------
 arch/powerpc/kvm/book3s_hv_rmhandlers.S |   37 +++++++++++++++----------------
 arch/powerpc/kvm/powerpc.c              |    7 ++++++
 include/uapi/linux/kvm.h                |    1 +
 6 files changed, 31 insertions(+), 28 deletions(-)

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 827a38d..8a652ba 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -243,6 +243,7 @@ struct kvm_arch {
 	int hpt_cma_alloc;
 	struct dentry *debugfs_dir;
 	struct dentry *htab_dentry;
+	u8 fwnmi_enabled;
 #endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
 	struct mutex hpt_mutex;
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 221d584..6a4e81a 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -506,6 +506,7 @@ int main(void)
 	DEFINE(KVM_ENABLED_HCALLS, offsetof(struct kvm, arch.enabled_hcalls));
 	DEFINE(KVM_LPCR, offsetof(struct kvm, arch.lpcr));
 	DEFINE(KVM_VRMA_SLB_V, offsetof(struct kvm, arch.vrma_slb_v));
+	DEFINE(KVM_FWNMI, offsetof(struct kvm, arch.fwnmi_enabled));
 	DEFINE(VCPU_DSISR, offsetof(struct kvm_vcpu, arch.shregs.dsisr));
 	DEFINE(VCPU_DAR, offsetof(struct kvm_vcpu, arch.shregs.dar));
 	DEFINE(VCPU_VPA, offsetof(struct kvm_vcpu, arch.vpa.pinned_addr));
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 2280497..1b1dff0 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -859,15 +859,9 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		r = RESUME_GUEST;
 		break;
 	case BOOK3S_INTERRUPT_MACHINE_CHECK:
-		/*
-		 * Deliver a machine check interrupt to the guest.
-		 * We have to do this, even if the host has handled the
-		 * machine check, because machine checks use SRR0/1 and
-		 * the interrupt might have trashed guest state in them.
-		 */
-		kvmppc_book3s_queue_irqprio(vcpu,
-					    BOOK3S_INTERRUPT_MACHINE_CHECK);
-		r = RESUME_GUEST;
+		/* Exit to guest with KVM_EXIT_NMI as exit reason */
+		run->exit_reason = KVM_EXIT_NMI;
+		r = RESUME_HOST;
 		break;
 	case BOOK3S_INTERRUPT_PROGRAM:
 	{
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index b98889e..f43c124 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -147,7 +147,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	addi	r1, r1, 112
 	ld	r7, HSTATE_HOST_MSR(r13)
 
-	cmpwi	cr1, r12, BOOK3S_INTERRUPT_MACHINE_CHECK
 	cmpwi	r12, BOOK3S_INTERRUPT_EXTERNAL
 	beq	11f
 	cmpwi	cr2, r12, BOOK3S_INTERRUPT_HMI
@@ -160,7 +159,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	mtmsrd	r6, 1			/* Clear RI in MSR */
 	mtsrr0	r8
 	mtsrr1	r7
-	beq	cr1, 13f		/* machine check */
 	RFI
 
 	/* On POWER7, we have external interrupts set to use HSRR0/1 */
@@ -168,8 +166,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	mtspr	SPRN_HSRR1, r7
 	ba	0x500
 
-13:	b	machine_check_fwnmi
-
 14:	mtspr	SPRN_HSRR0, r8
 	mtspr	SPRN_HSRR1, r7
 	b	hmi_exception_after_realmode
@@ -2381,24 +2377,27 @@ machine_check_realmode:
 	ld	r9, HSTATE_KVM_VCPU(r13)
 	li	r12, BOOK3S_INTERRUPT_MACHINE_CHECK
 	/*
-	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest through
-	 * machine check interrupt (set HSRR0 to 0x200). And for handled
-	 * errors (no-fatal), just go back to guest execution with current
-	 * HSRR0 instead of exiting guest. This new approach will inject
-	 * machine check to guest for fatal error causing guest to crash.
-	 *
-	 * 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.
+	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest
+	 * by exiting the guest with KVM_EXIT_NMI exit reason (exit
+	 * reason set later based on trap). For handled errors
+	 * (no-fatal), go back to guest execution with current HSRR0
+	 * instead of exiting the guest. This approach will cause
+	 * the guest to exit in case of fatal machine check error.
 	 */
-	ld	r10, VCPU_PC(r9)
-	ld	r11, VCPU_MSR(r9)
-	bne	2f	/* Continue guest execution. */
-	/* If not, deliver a machine check.  SRR0/1 are already set */
-	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
+	bne	2f	/* Continue guest execution? */
+	/* If not, check if guest is capable of handling NMI exit */
+	ld	r3, VCPU_KVM(r9)
+	ld	r3, KVM_FWNMI(r3)
+	cmpdi	r3, 1		/* FWNMI capable? */
+	bne	1f	/* Deliver machine check via guest's 0x200 vector */
+	b	mc_cont
+1:	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
 	ld	r11, VCPU_MSR(r9)
 	bl	kvmppc_msr_interrupt
-2:	b	fast_interrupt_c_return
+	b	fast_interrupt_c_return
+2:	ld	r10, VCPU_PC(r9)
+	ld	r11, VCPU_MSR(r9)
+	b	fast_interrupt_c_return
 
 /*
  * Check the reason we woke from nap, and take appropriate action.
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 2e51289..3e57636 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -567,6 +567,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = 1;
 		break;
 #endif
+	case KVM_CAP_PPC_FWNMI:
+		r = 1;
+		break;
 	default:
 		r = 0;
 		break;
@@ -1129,6 +1132,10 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 		break;
 	}
 #endif /* CONFIG_KVM_XICS */
+	case KVM_CAP_PPC_FWNMI:
+		r = 0;
+		vcpu->kvm->arch.fwnmi_enabled = true;
+		break;
 	default:
 		r = -EINVAL;
 		break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index a9256f0..2daf4b4 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -824,6 +824,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_MULTI_ADDRESS_SPACE 118
 #define KVM_CAP_GUEST_DEBUG_HW_BPS 119
 #define KVM_CAP_GUEST_DEBUG_HW_WPS 120
+#define KVM_CAP_PPC_FWNMI 121
 
 #ifdef KVM_CAP_IRQ_ROUTING