KVM: MIPS: Use CP0_BadInstr[P] for emulation

When exiting from the guest, store the values of the CP0_BadInstr and
CP0_BadInstrP registers if they exist, which contain the encodings of
the instructions which caused the last synchronous exception.

When the instruction is needed for emulation, kvm_get_badinstr() and
kvm_get_badinstrp() are used instead of calling kvm_get_inst() directly,
to decide whether to read the saved CP0_BadInstr/CP0_BadInstrP registers
(if they exist), or read the instruction from memory (if not).

The use of these registers should be more robust than using
kvm_get_inst(), as it actually gives the instruction encoding seen by
the hardware rather than relying on user accessors after the fact, which
can be fooled by incoherent icache or a racing code modification. It
will also work with VZ, where the guest virtual memory isn't directly
accessible by the host with user accessors.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org

1 files changed, 45 insertions, 3 deletions

diff --git a/arch/mips/kvm/emulate.c b/arch/mips/kvm/emulate.c
index b906fc0589f3..b295a4a1496f 100644
--- a/arch/mips/kvm/emulate.c
+++ b/arch/mips/kvm/emulate.c
@@ -54,7 +54,7 @@ static int kvm_compute_return_epc(struct kvm_vcpu *vcpu, unsigned long instpc,
 	}
 
 	/* Read the instruction */
-	err = kvm_get_inst((u32 *)epc, vcpu, &insn.word);
+	err = kvm_get_badinstrp((u32 *)epc, vcpu, &insn.word);
 	if (err)
 		return err;
 
@@ -259,6 +259,48 @@ enum emulation_result update_pc(struct kvm_vcpu *vcpu, u32 cause)
 }
 
 /**
+ * kvm_get_badinstr() - Get bad instruction encoding.
+ * @opc:	Guest pointer to faulting instruction.
+ * @vcpu:	KVM VCPU information.
+ *
+ * Gets the instruction encoding of the faulting instruction, using the saved
+ * BadInstr register value if it exists, otherwise falling back to reading guest
+ * memory at @opc.
+ *
+ * Returns:	The instruction encoding of the faulting instruction.
+ */
+int kvm_get_badinstr(u32 *opc, struct kvm_vcpu *vcpu, u32 *out)
+{
+	if (cpu_has_badinstr) {
+		*out = vcpu->arch.host_cp0_badinstr;
+		return 0;
+	} else {
+		return kvm_get_inst(opc, vcpu, out);
+	}
+}
+
+/**
+ * kvm_get_badinstrp() - Get bad prior instruction encoding.
+ * @opc:	Guest pointer to prior faulting instruction.
+ * @vcpu:	KVM VCPU information.
+ *
+ * Gets the instruction encoding of the prior faulting instruction (the branch
+ * containing the delay slot which faulted), using the saved BadInstrP register
+ * value if it exists, otherwise falling back to reading guest memory at @opc.
+ *
+ * Returns:	The instruction encoding of the prior faulting instruction.
+ */
+int kvm_get_badinstrp(u32 *opc, struct kvm_vcpu *vcpu, u32 *out)
+{
+	if (cpu_has_badinstrp) {
+		*out = vcpu->arch.host_cp0_badinstrp;
+		return 0;
+	} else {
+		return kvm_get_inst(opc, vcpu, out);
+	}
+}
+
+/**
  * kvm_mips_count_disabled() - Find whether the CP0_Count timer is disabled.
  * @vcpu:	Virtual CPU.
  *
@@ -1839,7 +1881,7 @@ enum emulation_result kvm_mips_emulate_inst(u32 cause, u32 *opc,
 	/* Fetch the instruction. */
 	if (cause & CAUSEF_BD)
 		opc += 1;
-	err = kvm_get_inst(opc, vcpu, &inst.word);
+	err = kvm_get_badinstr(opc, vcpu, &inst.word);
 	if (err)
 		return EMULATE_FAIL;
 
@@ -2434,7 +2476,7 @@ enum emulation_result kvm_mips_handle_ri(u32 cause, u32 *opc,
 	/* Fetch the instruction. */
 	if (cause & CAUSEF_BD)
 		opc += 1;
-	err = kvm_get_inst(opc, vcpu, &inst.word);
+	err = kvm_get_badinstr(opc, vcpu, &inst.word);
 	if (err) {
 		kvm_err("%s: Cannot get inst @ %p (%d)\n", __func__, opc, err);
 		return EMULATE_FAIL;