KVM: PPC: Book3S HV P9: Optimise timebase reads

Reduce the number of mfTB executed by passing the current timebase
around entry and exit code rather than read it multiple times.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-25-npiggin@gmail.com

arch/powerpc/include/asm/kvm_book3s_64.h

@@ -154,7 +154,7 @@ static inline bool kvmhv_vcpu_is_radix(struct kvm_vcpu *vcpu)
 	return radix;
 }
 
-int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpcr);
+int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpcr, u64 *tb);
 
 #define KVM_DEFAULT_HPT_ORDER	24	/* 16MB HPT by default */
 #endif

arch/powerpc/kvm/book3s_hv_p9_entry.c

@@ -183,13 +183,13 @@ static void save_clear_guest_mmu(struct kvm *kvm, struct kvm_vcpu *vcpu)
 	}
 }
 
-int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpcr)
+int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpcr, u64 *tb)
 {
 	struct kvm *kvm = vcpu->kvm;
 	struct kvm_nested_guest *nested = vcpu->arch.nested;
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 	s64 hdec, dec;
-	u64 tb, purr, spurr;
+	u64 purr, spurr;
 	u64 *exsave;
 	bool ri_set;
 	int trap;
@@ -203,8 +203,7 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 	unsigned long host_dawr1;
 	unsigned long host_dawrx1;
 
-	tb = mftb();
-	hdec = time_limit - tb;
+	hdec = time_limit - *tb;
 	if (hdec < 0)
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
 
@@ -230,11 +229,13 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 	}
 
 	if (vc->tb_offset) {
-		u64 new_tb = tb + vc->tb_offset;
+		u64 new_tb = *tb + vc->tb_offset;
 		mtspr(SPRN_TBU40, new_tb);
-		tb = mftb();
-		if ((tb & 0xffffff) < (new_tb & 0xffffff))
-			mtspr(SPRN_TBU40, new_tb + 0x1000000);
+		if ((mftb() & 0xffffff) < (new_tb & 0xffffff)) {
+			new_tb += 0x1000000;
+			mtspr(SPRN_TBU40, new_tb);
+		}
+		*tb = new_tb;
 		vc->tb_offset_applied = vc->tb_offset;
 	}
 
@@ -317,7 +318,7 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 	 */
 	mtspr(SPRN_HDEC, hdec);
 
-	mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
+	mtspr(SPRN_DEC, vcpu->arch.dec_expires - *tb);
 
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
 tm_return_to_guest:
@@ -466,15 +467,17 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 	dec = mfspr(SPRN_DEC);
 	if (!(lpcr & LPCR_LD)) /* Sign extend if not using large decrementer */
 		dec = (s32) dec;
-	tb = mftb();
-	vcpu->arch.dec_expires = dec + tb;
+	*tb = mftb();
+	vcpu->arch.dec_expires = dec + *tb;
 
 	if (vc->tb_offset_applied) {
-		u64 new_tb = tb - vc->tb_offset_applied;
+		u64 new_tb = *tb - vc->tb_offset_applied;
 		mtspr(SPRN_TBU40, new_tb);
-		tb = mftb();
-		if ((tb & 0xffffff) < (new_tb & 0xffffff))
-			mtspr(SPRN_TBU40, new_tb + 0x1000000);
+		if ((mftb() & 0xffffff) < (new_tb & 0xffffff)) {
+			new_tb += 0x1000000;
+			mtspr(SPRN_TBU40, new_tb);
+		}
+		*tb = new_tb;
 		vc->tb_offset_applied = 0;
 	}