Revert "KVM: PPC: Book3S HV Nested: Stop forwarding all HFUs to L1"

This reverts commit 7c3ded5.

On PowerNV, when a nested guest tries to use a feature prohibited by
HFSCR, the nested hypervisor (L1) should get a H_FAC_UNAVAILABLE trap
so that L1 can emulate the feature. But with the change introduced by
commit 7c3ded5 ("KVM: PPC: Book3S HV Nested: Stop forwarding all HFUs
to L1") the L1 ends up getting a H_EMUL_ASSIST because of which, the L1
ends up injecting a SIGILL when L2 (nested guest) tries to use doorbells.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241109063301.105289-2-gautam@linux.ibm.com

Author: Gautam Menghani

arch/powerpc/kvm/book3s_hv.c

@@ -2063,36 +2063,9 @@ static int kvmppc_handle_nested_exit(struct kvm_vcpu *vcpu)
 		fallthrough; /* go to facility unavailable handler */
 #endif
 
-	case BOOK3S_INTERRUPT_H_FAC_UNAVAIL: {
-		u64 cause = vcpu->arch.hfscr >> 56;
-
-		/*
-		 * Only pass HFU interrupts to the L1 if the facility is
-		 * permitted but disabled by the L1's HFSCR, otherwise
-		 * the interrupt does not make sense to the L1 so turn
-		 * it into a HEAI.
-		 */
-		if (!(vcpu->arch.hfscr_permitted & (1UL << cause)) ||
-				(vcpu->arch.nested_hfscr & (1UL << cause))) {
-			ppc_inst_t pinst;
-			vcpu->arch.trap = BOOK3S_INTERRUPT_H_EMUL_ASSIST;
-
-			/*
-			 * If the fetch failed, return to guest and
-			 * try executing it again.
-			 */
-			r = kvmppc_get_last_inst(vcpu, INST_GENERIC, &pinst);
-			vcpu->arch.emul_inst = ppc_inst_val(pinst);
-			if (r != EMULATE_DONE)
-				r = RESUME_GUEST;
-			else
-				r = RESUME_HOST;
-		} else {
-			r = RESUME_HOST;
-		}
-
+	case BOOK3S_INTERRUPT_H_FAC_UNAVAIL:
+		r = RESUME_HOST;
 		break;
-	}
 
 	case BOOK3S_INTERRUPT_HV_RM_HARD:
 		vcpu->arch.trap = 0;