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>

1 files changed, 38 insertions, 1 deletions

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: