KVM: PPC: PR: Handle FSCR feature deselects

We handle FSCR feature bits (well, TAR only really today) lazily when the guest
starts using them. So when a guest activates the bit and later uses that feature
we enable it for real in hardware.

However, when the guest stops using that bit we don't stop setting it in
hardware. That means we can potentially lose a trap that the guest expects to
happen because it thinks a feature is not active.

This patch adds support to drop TAR when then guest turns it off in FSCR. While
at it it also restricts FSCR access to 64bit systems - 32bit ones don't have it.

Signed-off-by: Alexander Graf <[e-mail]>

arch/powerpc/include/asm/kvm_book3s.h

···
 			struct kvm_memory_slot *memslot, unsigned long *map);
 extern void kvmppc_update_lpcr(struct kvm *kvm, unsigned long lpcr,
 			unsigned long mask);
+extern void kvmppc_set_fscr(struct kvm_vcpu *vcpu, u64 fscr);
 
 extern void kvmppc_entry_trampoline(void);
 extern void kvmppc_hv_entry_trampoline(void);

arch/powerpc/kvm/book3s_emulate.c

···
 	case SPRN_GQR7:
 		to_book3s(vcpu)->gqr[sprn - SPRN_GQR0] = spr_val;
 		break;
-	case SPRN_FSCR:
-		vcpu->arch.fscr = spr_val;
-		break;
 #ifdef CONFIG_PPC_BOOK3S_64
+	case SPRN_FSCR:
+		kvmppc_set_fscr(vcpu, spr_val);
+		break;
 	case SPRN_BESCR:
 		vcpu->arch.bescr = spr_val;
 		break;
···
 	case SPRN_GQR7:
 		*spr_val = to_book3s(vcpu)->gqr[sprn - SPRN_GQR0];
 		break;
+#ifdef CONFIG_PPC_BOOK3S_64
 	case SPRN_FSCR:
 		*spr_val = vcpu->arch.fscr;
 		break;
-#ifdef CONFIG_PPC_BOOK3S_64
 	case SPRN_BESCR:
 		*spr_val = vcpu->arch.bescr;
 		break;

arch/powerpc/kvm/book3s_pr.c

···
 
 	return RESUME_GUEST;
 }
+
+void kvmppc_set_fscr(struct kvm_vcpu *vcpu, u64 fscr)
+{
+	if ((vcpu->arch.fscr & FSCR_TAR) && !(fscr & FSCR_TAR)) {
+		/* TAR got dropped, drop it in shadow too */
+		kvmppc_giveup_fac(vcpu, FSCR_TAR_LG);
+	}
+	vcpu->arch.fscr = fscr;
+}
 #endif
 
 int kvmppc_handle_exit_pr(struct kvm_run *run, struct kvm_vcpu *vcpu,