arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0

CNTPCTSS_EL0 and CNTVCTSS_EL0 are alternatives to the usual
CNTPCT_EL0 and CNTVCT_EL0 that do not require a previous ISB
to be synchronised (SS stands for Self-Synchronising).

Use the ARM64_HAS_ECV capability to control alternative sequences
that switch to these low(er)-cost primitives. Note that the
counter access in the VDSO is for now left alone until we decide
whether we want to allow this.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211017124225.3018098-16-maz@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/arch_timer.h

@@ -64,14 +64,26 @@ DECLARE_PER_CPU(const struct arch_timer_erratum_workaround *,
 
 static inline notrace u64 arch_timer_read_cntpct_el0(void)
 {
-	isb();
-	return read_sysreg(cntpct_el0);
+	u64 cnt;
+
+	asm volatile(ALTERNATIVE("isb\n mrs %0, cntpct_el0",
+				 "nop\n" __mrs_s("%0", SYS_CNTPCTSS_EL0),
+				 ARM64_HAS_ECV)
+		     : "=r" (cnt));
+
+	return cnt;
 }
 
 static inline notrace u64 arch_timer_read_cntvct_el0(void)
 {
-	isb();
-	return read_sysreg(cntvct_el0);
+	u64 cnt;
+
+	asm volatile(ALTERNATIVE("isb\n mrs %0, cntvct_el0",
+				 "nop\n" __mrs_s("%0", SYS_CNTVCTSS_EL0),
+				 ARM64_HAS_ECV)
+		     : "=r" (cnt));
+
+	return cnt;
 }
 
 #define arch_timer_reg_read_stable(reg)					\
@@ -174,8 +186,10 @@ static __always_inline u64 __arch_counter_get_cntpct(void)
 {
 	u64 cnt;
 
-	isb();
-	cnt = read_sysreg(cntpct_el0);
+	asm volatile(ALTERNATIVE("isb\n mrs %0, cntpct_el0",
+				 "nop\n" __mrs_s("%0", SYS_CNTPCTSS_EL0),
+				 ARM64_HAS_ECV)
+		     : "=r" (cnt));
 	arch_counter_enforce_ordering(cnt);
 	return cnt;
 }
@@ -193,8 +207,10 @@ static __always_inline u64 __arch_counter_get_cntvct(void)
 {
 	u64 cnt;
 
-	isb();
-	cnt = read_sysreg(cntvct_el0);
+	asm volatile(ALTERNATIVE("isb\n mrs %0, cntvct_el0",
+				 "nop\n" __mrs_s("%0", SYS_CNTVCTSS_EL0),
+				 ARM64_HAS_ECV)
+		     : "=r" (cnt));
 	arch_counter_enforce_ordering(cnt);
 	return cnt;
 }

arch/arm64/include/asm/sysreg.h

@@ -507,6 +507,9 @@
 
 #define SYS_CNTFRQ_EL0			sys_reg(3, 3, 14, 0, 0)
 
+#define SYS_CNTPCTSS_EL0		sys_reg(3, 3, 14, 0, 5)
+#define SYS_CNTVCTSS_EL0		sys_reg(3, 3, 14, 0, 6)
+
 #define SYS_CNTP_TVAL_EL0		sys_reg(3, 3, 14, 2, 0)
 #define SYS_CNTP_CTL_EL0		sys_reg(3, 3, 14, 2, 1)
 #define SYS_CNTP_CVAL_EL0		sys_reg(3, 3, 14, 2, 2)