KVM: PPC: Book3S HV P9: Remove subcore HMI handling

On POWER9 and newer, rather than the complex HMI synchronisation and
subcore state, have each thread un-apply the guest TB offset before
calling into the early HMI handler.

This allows the subcore state to be avoided, including subcore enter
/ exit guest, which includes an expensive divide that shows up
slightly in profiles.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-54-npiggin@gmail.com

arch/powerpc/include/asm/kvm_ppc.h

@@ -759,6 +759,7 @@ void kvmppc_realmode_machine_check(struct kvm_vcpu *vcpu);
 void kvmppc_subcore_enter_guest(void);
 void kvmppc_subcore_exit_guest(void);
 long kvmppc_realmode_hmi_handler(void);
+long kvmppc_p9_realmode_hmi_handler(struct kvm_vcpu *vcpu);
 long kvmppc_h_enter(struct kvm_vcpu *vcpu, unsigned long flags,
                     long pte_index, unsigned long pteh, unsigned long ptel);
 long kvmppc_h_remove(struct kvm_vcpu *vcpu, unsigned long flags,

arch/powerpc/kvm/book3s_hv.c

@@ -4033,8 +4033,6 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	vcpu->arch.ceded = 0;
 
-	kvmppc_subcore_enter_guest();
-
 	vcpu_vpa_increment_dispatch(vcpu);
 
 	if (kvmhv_on_pseries()) {
@@ -4087,8 +4085,6 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	vcpu_vpa_increment_dispatch(vcpu);
 
-	kvmppc_subcore_exit_guest();
-
 	return trap;
 }
 
@@ -6102,9 +6098,11 @@ static int kvmppc_book3s_init_hv(void)
 	if (r)
 		return r;
 
-	r = kvm_init_subcore_bitmap();
-	if (r)
-		return r;
+	if (!cpu_has_feature(CPU_FTR_ARCH_300)) {
+		r = kvm_init_subcore_bitmap();
+		if (r)
+			return r;
+	}
 
 	/*
 	 * We need a way of accessing the XICS interrupt controller,

arch/powerpc/kvm/book3s_hv_hmi.c

@@ -20,10 +20,15 @@ void wait_for_subcore_guest_exit(void)
 
 	/*
 	 * NULL bitmap pointer indicates that KVM module hasn't
-	 * been loaded yet and hence no guests are running.
+	 * been loaded yet and hence no guests are running, or running
+	 * on POWER9 or newer CPU.
+	 *
 	 * If no KVM is in use, no need to co-ordinate among threads
 	 * as all of them will always be in host and no one is going
 	 * to modify TB other than the opal hmi handler.
+	 *
+	 * POWER9 and newer don't need this synchronisation.
+	 *
 	 * Hence, just return from here.
 	 */
 	if (!local_paca->sibling_subcore_state)

arch/powerpc/kvm/book3s_hv_p9_entry.c

@@ -1013,7 +1013,7 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 		kvmppc_realmode_machine_check(vcpu);
 
 	} else if (unlikely(trap == BOOK3S_INTERRUPT_HMI)) {
-		kvmppc_realmode_hmi_handler();
+		kvmppc_p9_realmode_hmi_handler(vcpu);
 
 	} else if (trap == BOOK3S_INTERRUPT_H_EMUL_ASSIST) {
 		vcpu->arch.emul_inst = mfspr(SPRN_HEIR);

arch/powerpc/kvm/book3s_hv_ras.c

@@ -136,6 +136,60 @@ void kvmppc_realmode_machine_check(struct kvm_vcpu *vcpu)
 	vcpu->arch.mce_evt = mce_evt;
 }
 
+
+long kvmppc_p9_realmode_hmi_handler(struct kvm_vcpu *vcpu)
+{
+	struct kvmppc_vcore *vc = vcpu->arch.vcore;
+	long ret = 0;
+
+	/*
+	 * Unapply and clear the offset first. That way, if the TB was not
+	 * resynced then it will remain in host-offset, and if it was resynced
+	 * then it is brought into host-offset. Then the tb offset is
+	 * re-applied before continuing with the KVM exit.
+	 *
+	 * This way, we don't need to actually know whether not OPAL resynced
+	 * the timebase or do any of the complicated dance that the P7/8
+	 * path requires.
+	 */
+	if (vc->tb_offset_applied) {
+		u64 new_tb = mftb() - vc->tb_offset_applied;
+		mtspr(SPRN_TBU40, new_tb);
+		if ((mftb() & 0xffffff) < (new_tb & 0xffffff)) {
+			new_tb += 0x1000000;
+			mtspr(SPRN_TBU40, new_tb);
+		}
+		vc->tb_offset_applied = 0;
+	}
+
+	local_paca->hmi_irqs++;
+
+	if (hmi_handle_debugtrig(NULL) >= 0) {
+		ret = 1;
+		goto out;
+	}
+
+	if (ppc_md.hmi_exception_early)
+		ppc_md.hmi_exception_early(NULL);
+
+out:
+	if (vc->tb_offset) {
+		u64 new_tb = mftb() + vc->tb_offset;
+		mtspr(SPRN_TBU40, new_tb);
+		if ((mftb() & 0xffffff) < (new_tb & 0xffffff)) {
+			new_tb += 0x1000000;
+			mtspr(SPRN_TBU40, new_tb);
+		}
+		vc->tb_offset_applied = vc->tb_offset;
+	}
+
+	return ret;
+}
+
+/*
+ * The following subcore HMI handling is all only for pre-POWER9 CPUs.
+ */
+
 /* Check if dynamic split is in force and return subcore size accordingly. */
 static inline int kvmppc_cur_subcore_size(void)
 {