KVM: PPC: Book3S PR: Make instruction fetch fallback work for system calls

It turns out that if we exit the guest due to a hcall instruction (sc 1),
and the loading of the instruction in the guest exit path fails for any
reason, the call to kvmppc_ld() in kvmppc_get_last_inst() fetches the
instruction after the hcall instruction rather than the hcall itself.
This in turn means that the instruction doesn't get recognized as an
hcall in kvmppc_handle_exit_pr() but gets passed to the guest kernel
as a sc instruction.  That usually results in the guest kernel getting
a return code of 38 (ENOSYS) from an hcall, which often triggers a
BUG_ON() or other failure.

This fixes the problem by adding a new variant of kvmppc_get_last_inst()
called kvmppc_get_last_sc(), which fetches the instruction if necessary
from pc - 4 rather than pc.

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

arch/powerpc/include/asm/kvm_book3s.h

@@ -334,6 +334,27 @@ static inline u32 kvmppc_get_last_inst(struct kvm_vcpu *vcpu)
 	return r;
 }
 
+/*
+ * Like kvmppc_get_last_inst(), but for fetching a sc instruction.
+ * Because the sc instruction sets SRR0 to point to the following
+ * instruction, we have to fetch from pc - 4.
+ */
+static inline u32 kvmppc_get_last_sc(struct kvm_vcpu *vcpu)
+{
+	ulong pc = kvmppc_get_pc(vcpu) - 4;
+	struct kvmppc_book3s_shadow_vcpu *svcpu = svcpu_get(vcpu);
+	u32 r;
+
+	/* Load the instruction manually if it failed to do so in the
+	 * exit path */
+	if (svcpu->last_inst == KVM_INST_FETCH_FAILED)
+		kvmppc_ld(vcpu, &pc, sizeof(u32), &svcpu->last_inst, false);
+
+	r = svcpu->last_inst;
+	svcpu_put(svcpu);
+	return r;
+}
+
 static inline ulong kvmppc_get_fault_dar(struct kvm_vcpu *vcpu)
 {
 	struct kvmppc_book3s_shadow_vcpu *svcpu = svcpu_get(vcpu);
@@ -446,6 +467,23 @@ static inline u32 kvmppc_get_last_inst(struct kvm_vcpu *vcpu)
 	return vcpu->arch.last_inst;
 }
 
+/*
+ * Like kvmppc_get_last_inst(), but for fetching a sc instruction.
+ * Because the sc instruction sets SRR0 to point to the following
+ * instruction, we have to fetch from pc - 4.
+ */
+static inline u32 kvmppc_get_last_sc(struct kvm_vcpu *vcpu)
+{
+	ulong pc = kvmppc_get_pc(vcpu) - 4;
+
+	/* Load the instruction manually if it failed to do so in the
+	 * exit path */
+	if (vcpu->arch.last_inst == KVM_INST_FETCH_FAILED)
+		kvmppc_ld(vcpu, &pc, sizeof(u32), &vcpu->arch.last_inst, false);
+
+	return vcpu->arch.last_inst;
+}
+
 static inline ulong kvmppc_get_fault_dar(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.fault_dar;

arch/powerpc/kvm/book3s_pr.c

@@ -792,7 +792,7 @@ program_interrupt:
 	}
 	case BOOK3S_INTERRUPT_SYSCALL:
 		if (vcpu->arch.papr_enabled &&
-		    (kvmppc_get_last_inst(vcpu) == 0x44000022) &&
+		    (kvmppc_get_last_sc(vcpu) == 0x44000022) &&
 		    !(vcpu->arch.shared->msr & MSR_PR)) {
 			/* SC 1 papr hypercalls */
 			ulong cmd = kvmppc_get_gpr(vcpu, 3);