[PATCH v3 10/11] KVM: arm64: Do not perform an isb() if ZCR_EL2 isn't updated

[Fuad Tabba]

When conditionally updating ZCR, no need to perform an isb() if
the value isn't updated. Introduce and use
sve_cond_update_zcr_vq_isb(), which only has the barrier if the
value of ZCR is updated.

Signed-off-by: Fuad Tabba <tabba at google.com>
---
This patch is undone by the following patch. Please consider one
of these two patches, or feel free to drop both.
---
 arch/arm64/include/asm/fpsimd.h         | 14 ++++++++++++--
 arch/arm64/kvm/hyp/include/hyp/switch.h |  3 +--
 arch/arm64/kvm/hyp/nvhe/hyp-main.c      |  3 +--
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index bc69ac368d73..531f805e4643 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -219,15 +219,24 @@ static inline void sve_user_enable(void)
 	sysreg_clear_set(cpacr_el1, 0, CPACR_EL1_ZEN_EL0EN);
 }
 
-#define sve_cond_update_zcr_vq(val, reg)		\
+#define __sve_cond_update_zcr_vq(val, reg, sync)	\
 	do {						\
 		u64 __zcr = read_sysreg_s((reg));	\
 		u64 __new = __zcr & ~ZCR_ELx_LEN_MASK;	\
 		__new |= (val) & ZCR_ELx_LEN_MASK;	\
-		if (__zcr != __new)			\
+		if (__zcr != __new) {			\
 			write_sysreg_s(__new, (reg));	\
+			if (sync)			\
+				isb();			\
+		}					\
 	} while (0)
 
+#define sve_cond_update_zcr_vq(val, reg)		\
+	__sve_cond_update_zcr_vq(val, reg, false)
+
+#define sve_cond_update_zcr_vq_isb(val, reg)		\
+	__sve_cond_update_zcr_vq(val, reg, true)
+
 /*
  * Probing and setup functions.
  * Calls to these functions must be serialised with one another.
@@ -330,6 +339,7 @@ static inline void sve_user_disable(void) { BUILD_BUG(); }
 static inline void sve_user_enable(void) { BUILD_BUG(); }
 
 #define sve_cond_update_zcr_vq(val, reg) do { } while (0)
+#define sve_cond_update_zcr_vq_isb(val, reg) do { } while (0)
 
 static inline void vec_init_vq_map(enum vec_type t) { }
 static inline void vec_update_vq_map(enum vec_type t) { }
diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 24b43f1f3d51..162a60bcc27d 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -314,8 +314,7 @@ static bool kvm_hyp_handle_mops(struct kvm_vcpu *vcpu, u64 *exit_code)
 
 static inline void __hyp_sve_restore_guest(struct kvm_vcpu *vcpu)
 {
-	sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1, SYS_ZCR_EL2);
-	isb();
+	sve_cond_update_zcr_vq_isb(vcpu_sve_max_vq(vcpu) - 1, SYS_ZCR_EL2);
 	__sve_restore_state(vcpu_sve_pffr(vcpu),
 			    &vcpu->arch.ctxt.fp_regs.fpsr);
 	write_sysreg_el1(__vcpu_sys_reg(vcpu, ZCR_EL1), SYS_ZCR);
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index b28d7d8cdc30..cef51fe80aa8 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -31,8 +31,7 @@ static void __hyp_sve_save_guest(struct kvm_vcpu *vcpu)
 	 * the guest. The layout of the data when saving the sve state depends
 	 * on the VL, so use a consistent (i.e., the maximum) guest VL.
 	 */
-	sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1, SYS_ZCR_EL2);
-	isb();
+	sve_cond_update_zcr_vq_isb(vcpu_sve_max_vq(vcpu) - 1, SYS_ZCR_EL2);
 	__sve_save_state(vcpu_sve_pffr(vcpu), &vcpu->arch.ctxt.fp_regs.fpsr);
 	write_sysreg_s(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
 }
-- 
2.45.1.288.g0e0cd299f1-goog