arm64: sve: Provide a conditional update accessor for ZCR_ELx

A common pattern is to conditionally update ZCR_ELx in order
to avoid the "self-synchronizing" effect that writing to this
register has.

Let's provide an accessor that does exactly this.
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/fpsimd.h

@@ -130,6 +130,15 @@ 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)		\
+	do {						\
+		u64 __zcr = read_sysreg_s((reg));	\
+		u64 __new = __zcr & ~ZCR_ELx_LEN_MASK;	\
+		__new |= (val) & ZCR_ELx_LEN_MASK;	\
+		if (__zcr != __new)			\
+			write_sysreg_s(__new, (reg));	\
+	} while (0)
+
 /*
  * Probing and setup functions.
  * Calls to these functions must be serialised with one another.