KVM: PPC: Book3S HV: Better handling of exceptions that happen in real mode

When an interrupt or exception happens in the guest that comes to the
host, the CPU goes to hypervisor real mode (MMU off) to handle the
exception but doesn't change the MMU context.  After saving a few
registers, we then clear the "in guest" flag.  If, for any reason,
we get an exception in the real-mode code, that then gets handled
by the normal kernel exception handlers, which turn the MMU on.  This
is disastrous if the MMU is still set to the guest context, since we
end up executing instructions from random places in the guest kernel
with hypervisor privilege.

In order to catch this situation, we define a new value for the "in guest"
flag, KVM_GUEST_MODE_HOST_HV, to indicate that we are in hypervisor real
mode with guest MMU context.  If the "in guest" flag is set to this value,
we branch off to an emergency handler.  For the moment, this just does
a branch to self to stop the CPU from doing anything further.

While we're here, we define another new flag value to indicate that we
are in a HV guest, as distinct from a PR guest.  This will be useful
when we have a kernel that can support both PR and HV guests concurrently.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

2 files changed, 27 insertions, 6 deletions

diff --git a/arch/powerpc/include/asm/kvm_asm.h b/arch/powerpc/include/asm/kvm_asm.h
index e2d4d467ee93..1bd92fd43cfb 100644
--- a/arch/powerpc/include/asm/kvm_asm.h
+++ b/arch/powerpc/include/asm/kvm_asm.h
@@ -138,6 +138,8 @@
 #define KVM_GUEST_MODE_NONE	0
 #define KVM_GUEST_MODE_GUEST	1
 #define KVM_GUEST_MODE_SKIP	2
+#define KVM_GUEST_MODE_GUEST_HV	3
+#define KVM_GUEST_MODE_HOST_HV	4
 
 #define KVM_INST_FETCH_FAILED	-1
 
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index 6e3370f42a63..84105eb18a0e 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -387,6 +387,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_206)
 	mtspr	SPRN_DAR, r5
 	mtspr	SPRN_DSISR, r6
 
+	li	r6, KVM_GUEST_MODE_HOST_HV
+	stb	r6, HSTATE_IN_GUEST(r13)
+
 BEGIN_FTR_SECTION
 	/* Restore AMR and UAMOR, set AMOR to all 1s */
 	ld	r5,VCPU_AMR(r4)
@@ -690,7 +693,7 @@ fast_guest_return:
 	mtspr	SPRN_HSRR1,r11
 
 	/* Activate guest mode, so faults get handled by KVM */
-	li	r9, KVM_GUEST_MODE_GUEST
+	li	r9, KVM_GUEST_MODE_GUEST_HV
 	stb	r9, HSTATE_IN_GUEST(r13)
 
 	/* Enter guest */
@@ -750,6 +753,14 @@ kvmppc_interrupt:
 	 */
 	/* abuse host_r2 as third scratch area; we get r2 from PACATOC(r13) */
 	std	r9, HSTATE_HOST_R2(r13)
+
+	lbz	r9, HSTATE_IN_GUEST(r13)
+	cmpwi	r9, KVM_GUEST_MODE_HOST_HV
+	beq	kvmppc_bad_host_intr
+	/* We're now back in the host but in guest MMU context */
+	li	r9, KVM_GUEST_MODE_HOST_HV
+	stb	r9, HSTATE_IN_GUEST(r13)
+
 	ld	r9, HSTATE_KVM_VCPU(r13)
 
 	/* Save registers */
@@ -801,10 +812,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	std	r3, VCPU_GPR(R13)(r9)
 	std	r4, VCPU_LR(r9)
 
-	/* Unset guest mode */
-	li	r0, KVM_GUEST_MODE_NONE
-	stb	r0, HSTATE_IN_GUEST(r13)
-
 	stw	r12,VCPU_TRAP(r9)
 
 	/* Save HEIR (HV emulation assist reg) in last_inst
@@ -1198,6 +1205,10 @@ BEGIN_FTR_SECTION
 	mtspr	SPRN_AMR,r6
 END_FTR_SECTION_IFSET(CPU_FTR_ARCH_206)
 
+	/* Unset guest mode */
+	li	r0, KVM_GUEST_MODE_NONE
+	stb	r0, HSTATE_IN_GUEST(r13)
+
 	/* Switch DSCR back to host value */
 BEGIN_FTR_SECTION
 	mfspr	r8, SPRN_DSCR
@@ -1400,7 +1411,7 @@ fast_interrupt_c_return:
 	stw	r8, VCPU_LAST_INST(r9)
 
 	/* Unset guest mode. */
-	li	r0, KVM_GUEST_MODE_NONE
+	li	r0, KVM_GUEST_MODE_HOST_HV
 	stb	r0, HSTATE_IN_GUEST(r13)
 	b	guest_exit_cont
 
@@ -1949,3 +1960,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
 	lwz	r7,VCPU_VRSAVE(r4)
 	mtspr	SPRN_VRSAVE,r7
 	blr
+
+/*
+ * We come here if we get any exception or interrupt while we are
+ * executing host real mode code while in guest MMU context.
+ * For now just spin, but we should do something better.
+ */
+kvmppc_bad_host_intr:
+	b	.