KVM: PPC: Implement 'skip instruction' mode

To fetch the last instruction we were interrupted on, we enable DR in early
exit code, where we are still in a very transitional phase between guest
and host state.

Most of the time this seemed to work, but another CPU can easily flush our
TLB and HTAB which makes us go in the Linux page fault handler which totally
breaks because we still use the guest's SLB entries.

To work around that, let's introduce a second KVM guest mode that defines
that whenever we get a trap, we don't call the Linux handler or go into
the KVM exit code, but just jump over the faulting instruction.

That way a potentially bad lwz doesn't trigger any faults and we can later
on interpret the invalid instruction we fetched as "fetch didn't work".

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

4 files changed, 59 insertions, 6 deletions

diff --git a/arch/powerpc/include/asm/kvm_asm.h b/arch/powerpc/include/asm/kvm_asm.h
index af2abe74f544..aadf2dd6f84e 100644
--- a/arch/powerpc/include/asm/kvm_asm.h
+++ b/arch/powerpc/include/asm/kvm_asm.h
@@ -97,4 +97,10 @@
 #define RESUME_HOST             RESUME_FLAG_HOST
 #define RESUME_HOST_NV          (RESUME_FLAG_HOST|RESUME_FLAG_NV)
 
+#define KVM_GUEST_MODE_NONE     0
+#define KVM_GUEST_MODE_GUEST    1
+#define KVM_GUEST_MODE_SKIP     2
+
+#define KVM_INST_FETCH_FAILED   -1
+
 #endif /* __POWERPC_KVM_ASM_H__ */
diff --git a/arch/powerpc/kvm/book3s_64_rmhandlers.S b/arch/powerpc/kvm/book3s_64_rmhandlers.S
index cd9f0b609e48..9ad1c2645d6f 100644
--- a/arch/powerpc/kvm/book3s_64_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_64_rmhandlers.S
@@ -49,7 +49,7 @@ kvmppc_trampoline_\intno:
         mfcr    r12
         stw     r12, PACA_KVM_SCRATCH1(r13)
         lbz     r12, PACA_KVM_IN_GUEST(r13)
-        cmpwi   r12, 0
+        cmpwi   r12, KVM_GUEST_MODE_NONE
         bne     ..kvmppc_handler_hasmagic_\intno
         /* No KVM guest? Then jump back to the Linux handler! */
         lwz     r12, PACA_KVM_SCRATCH1(r13)
@@ -60,6 +60,11 @@ kvmppc_trampoline_\intno:
 
         /* Now we know we're handling a KVM guest */
 ..kvmppc_handler_hasmagic_\intno:
+
+        /* Should we just skip the faulting instruction? */
+        cmpwi   r12, KVM_GUEST_MODE_SKIP
+        beq     kvmppc_handler_skip_ins
+
         /* Let's store which interrupt we're handling */
         li      r12, \intno
 
@@ -86,6 +91,38 @@ INTERRUPT_TRAMPOLINE	BOOK3S_INTERRUPT_ALTIVEC
 INTERRUPT_TRAMPOLINE    BOOK3S_INTERRUPT_VSX
 
 /*
+ * Bring us back to the faulting code, but skip the
+ * faulting instruction.
+ *
+ * This is a generic exit path from the interrupt
+ * trampolines above.
+ *
+ * Input Registers:
+ *
+ * R12               = free
+ * R13               = PACA
+ * PACA.KVM.SCRATCH0 = guest R12
+ * PACA.KVM.SCRATCH1 = guest CR
+ * SPRG_SCRATCH0     = guest R13
+ *
+ */
+kvmppc_handler_skip_ins:
+
+        /* Patch the IP to the next instruction */
+        mfsrr0  r12
+        addi    r12, r12, 4
+        mtsrr0  r12
+
+        /* Clean up all state */
+        lwz     r12, PACA_KVM_SCRATCH1(r13)
+        mtcr    r12
+        ld      r12, PACA_KVM_SCRATCH0(r13)
+        mfspr   r13, SPRN_SPRG_SCRATCH0
+
+        /* And get back into the code */
+        RFI
+
+/*
  * This trampoline brings us back to a real mode handler
  *
  * Input Registers:
diff --git a/arch/powerpc/kvm/book3s_64_slb.S b/arch/powerpc/kvm/book3s_64_slb.S
index 7188c11ed7d1..d07b88617b2c 100644
--- a/arch/powerpc/kvm/book3s_64_slb.S
+++ b/arch/powerpc/kvm/book3s_64_slb.S
@@ -212,10 +212,6 @@ kvmppc_handler_trampoline_exit:
         mfdar   r5
         mfdsisr r6
 
-        /* Unset guest state */
-        li      r9, 0
-        stb     r9, PACA_KVM_IN_GUEST(r13)
-
         /*
          * In order for us to easily get the last instruction,
          * we got the #vmexit at, we exploit the fact that the
@@ -233,18 +229,28 @@ kvmppc_handler_trampoline_exit:
 
 ld_last_inst:
         /* Save off the guest instruction we're at */
+
+        /* Set guest mode to 'jump over instruction' so if lwz faults
+         * we'll just continue at the next IP. */
+        li      r9, KVM_GUEST_MODE_SKIP
+        stb     r9, PACA_KVM_IN_GUEST(r13)
+
         /*    1) enable paging for data */
         mfmsr   r9
         ori     r11, r9, MSR_DR                 /* Enable paging for data */
         mtmsr   r11
         /*    2) fetch the instruction */
-        /* XXX implement PACA_KVM_IN_GUEST=2 path to safely jump over this */
+        li      r0, KVM_INST_FETCH_FAILED       /* In case lwz faults */
         lwz     r0, 0(r3)
         /*    3) disable paging again */
         mtmsr   r9
 
 no_ld_last_inst:
 
+        /* Unset guest mode */
+        li      r9, KVM_GUEST_MODE_NONE
+        stb     r9, PACA_KVM_IN_GUEST(r13)
+
         /* Restore bolted entries from the shadow and fix it along the way */
 
         /* We don't store anything in entry 0, so we don't need to take care of it */
diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
index 38219af0cd0e..04e317c1bbee 100644
--- a/arch/powerpc/kvm/emulate.c
+++ b/arch/powerpc/kvm/emulate.c
@@ -143,6 +143,10 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
 
         pr_debug(KERN_INFO "Emulating opcode %d / %d\n", get_op(inst), get_xop(inst));
 
+        /* Try again next time */
+        if (inst == KVM_INST_FETCH_FAILED)
+                return EMULATE_DONE;
+
         switch (get_op(inst)) {
         case OP_TRAP:
 #ifdef CONFIG_PPC64