KVM: PPC: Book3S HV P9: Only execute mtSPR if the value changed

Keep better track of the current SPR value in places where
they are to be loaded with a new context, to reduce expensive
mtSPR operations.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-27-npiggin@gmail.com

arch/powerpc/kvm/book3s_hv.c

@@ -4055,20 +4055,28 @@ static void switch_pmu_to_host(struct kvm_vcpu *vcpu,
 	}
 }
 
-static void load_spr_state(struct kvm_vcpu *vcpu)
+static void load_spr_state(struct kvm_vcpu *vcpu,
+				struct p9_host_os_sprs *host_os_sprs)
 {
-	mtspr(SPRN_DSCR, vcpu->arch.dscr);
-	mtspr(SPRN_IAMR, vcpu->arch.iamr);
-	mtspr(SPRN_PSPB, vcpu->arch.pspb);
-	mtspr(SPRN_FSCR, vcpu->arch.fscr);
 	mtspr(SPRN_TAR, vcpu->arch.tar);
 	mtspr(SPRN_EBBHR, vcpu->arch.ebbhr);
 	mtspr(SPRN_EBBRR, vcpu->arch.ebbrr);
 	mtspr(SPRN_BESCR, vcpu->arch.bescr);
+
 	if (cpu_has_feature(CPU_FTR_P9_TIDR))
 		mtspr(SPRN_TIDR, vcpu->arch.tid);
-	mtspr(SPRN_AMR, vcpu->arch.amr);
-	mtspr(SPRN_UAMOR, vcpu->arch.uamor);
+	if (host_os_sprs->iamr != vcpu->arch.iamr)
+		mtspr(SPRN_IAMR, vcpu->arch.iamr);
+	if (host_os_sprs->amr != vcpu->arch.amr)
+		mtspr(SPRN_AMR, vcpu->arch.amr);
+	if (vcpu->arch.uamor != 0)
+		mtspr(SPRN_UAMOR, vcpu->arch.uamor);
+	if (host_os_sprs->fscr != vcpu->arch.fscr)
+		mtspr(SPRN_FSCR, vcpu->arch.fscr);
+	if (host_os_sprs->dscr != vcpu->arch.dscr)
+		mtspr(SPRN_DSCR, vcpu->arch.dscr);
+	if (vcpu->arch.pspb != 0)
+		mtspr(SPRN_PSPB, vcpu->arch.pspb);
 
 	/*
 	 * DAR, DSISR, and for nested HV, SPRGs must be set with MSR[RI]
@@ -4083,20 +4091,21 @@ static void load_spr_state(struct kvm_vcpu *vcpu)
 
 static void store_spr_state(struct kvm_vcpu *vcpu)
 {
-	vcpu->arch.ctrl = mfspr(SPRN_CTRLF);
-
-	vcpu->arch.iamr = mfspr(SPRN_IAMR);
-	vcpu->arch.pspb = mfspr(SPRN_PSPB);
-	vcpu->arch.fscr = mfspr(SPRN_FSCR);
 	vcpu->arch.tar = mfspr(SPRN_TAR);
 	vcpu->arch.ebbhr = mfspr(SPRN_EBBHR);
 	vcpu->arch.ebbrr = mfspr(SPRN_EBBRR);
 	vcpu->arch.bescr = mfspr(SPRN_BESCR);
+
 	if (cpu_has_feature(CPU_FTR_P9_TIDR))
 		vcpu->arch.tid = mfspr(SPRN_TIDR);
+	vcpu->arch.iamr = mfspr(SPRN_IAMR);
 	vcpu->arch.amr = mfspr(SPRN_AMR);
 	vcpu->arch.uamor = mfspr(SPRN_UAMOR);
+	vcpu->arch.fscr = mfspr(SPRN_FSCR);
 	vcpu->arch.dscr = mfspr(SPRN_DSCR);
+	vcpu->arch.pspb = mfspr(SPRN_PSPB);
+
+	vcpu->arch.ctrl = mfspr(SPRN_CTRLF);
 }
 
 static void save_p9_host_os_sprs(struct p9_host_os_sprs *host_os_sprs)
@@ -4107,6 +4116,7 @@ static void save_p9_host_os_sprs(struct p9_host_os_sprs *host_os_sprs)
 	host_os_sprs->iamr = mfspr(SPRN_IAMR);
 	host_os_sprs->amr = mfspr(SPRN_AMR);
 	host_os_sprs->fscr = mfspr(SPRN_FSCR);
+	host_os_sprs->dscr = mfspr(SPRN_DSCR);
 }
 
 /* vcpu guest regs must already be saved */
@@ -4115,19 +4125,20 @@ static void restore_p9_host_os_sprs(struct kvm_vcpu *vcpu,
 {
 	mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
 
-	mtspr(SPRN_PSPB, 0);
-	mtspr(SPRN_UAMOR, 0);
-
-	mtspr(SPRN_DSCR, host_os_sprs->dscr);
 	if (cpu_has_feature(CPU_FTR_P9_TIDR))
 		mtspr(SPRN_TIDR, host_os_sprs->tidr);
-	mtspr(SPRN_IAMR, host_os_sprs->iamr);
-
+	if (host_os_sprs->iamr != vcpu->arch.iamr)
+		mtspr(SPRN_IAMR, host_os_sprs->iamr);
+	if (vcpu->arch.uamor != 0)
+		mtspr(SPRN_UAMOR, 0);
 	if (host_os_sprs->amr != vcpu->arch.amr)
 		mtspr(SPRN_AMR, host_os_sprs->amr);
-
 	if (host_os_sprs->fscr != vcpu->arch.fscr)
 		mtspr(SPRN_FSCR, host_os_sprs->fscr);
+	if (host_os_sprs->dscr != vcpu->arch.dscr)
+		mtspr(SPRN_DSCR, host_os_sprs->dscr);
+	if (vcpu->arch.pspb != 0)
+		mtspr(SPRN_PSPB, 0);
 
 	/* Save guest CTRL register, set runlatch to 1 */
 	if (!(vcpu->arch.ctrl & 1))
@@ -4219,7 +4230,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 #endif
 	mtspr(SPRN_VRSAVE, vcpu->arch.vrsave);
 
-	load_spr_state(vcpu);
+	load_spr_state(vcpu, &host_os_sprs);
 
 	if (kvmhv_on_pseries()) {
 		/*