arm64: Add HWCAP for self-synchronising virtual counter

Since userspace can make use of the CNTVSS_EL0 instruction, expose
it via a HWCAP.
Suggested-by: NWill Deacon <will@kernel.org>
Acked-by: NWill Deacon <will@kernel.org>
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211017124225.3018098-18-maz@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>

Documentation/arm64/cpu-feature-registers.rst

@@ -235,7 +235,15 @@ infrastructure:
      | DPB                          | [3-0]   |    y    |
      +------------------------------+---------+---------+
 
-  6) ID_AA64MMFR2_EL1 - Memory model feature register 2
+  6) ID_AA64MMFR0_EL1 - Memory model feature register 0
+
+     +------------------------------+---------+---------+
+     | Name                         |  bits   | visible |
+     +------------------------------+---------+---------+
+     | ECV                          | [63-60] |    y    |
+     +------------------------------+---------+---------+
+
+  7) ID_AA64MMFR2_EL1 - Memory model feature register 2
 
      +------------------------------+---------+---------+
      | Name                         |  bits   | visible |
@@ -243,7 +251,7 @@ infrastructure:
      | AT                           | [35-32] |    y    |
      +------------------------------+---------+---------+
 
-  7) ID_AA64ZFR0_EL1 - SVE feature ID register 0
+  8) ID_AA64ZFR0_EL1 - SVE feature ID register 0
 
      +------------------------------+---------+---------+
      | Name                         |  bits   | visible |

Documentation/arm64/elf_hwcaps.rst

@@ -247,6 +247,10 @@ HWCAP2_MTE
     Functionality implied by ID_AA64PFR1_EL1.MTE == 0b0010, as described
     by Documentation/arm64/memory-tagging-extension.rst.
 
+HWCAP2_ECV
+
+    Functionality implied by ID_AA64MMFR0_EL1.ECV == 0b0001.
+
 4. Unused AT_HWCAP bits
 -----------------------
 

arch/arm64/include/asm/hwcap.h

@@ -105,6 +105,7 @@
 #define KERNEL_HWCAP_RNG		__khwcap2_feature(RNG)
 #define KERNEL_HWCAP_BTI		__khwcap2_feature(BTI)
 #define KERNEL_HWCAP_MTE		__khwcap2_feature(MTE)
+#define KERNEL_HWCAP_ECV		__khwcap2_feature(ECV)
 
 /*
  * This yields a mask that user programs can use to figure out what

arch/arm64/include/uapi/asm/hwcap.h

@@ -75,5 +75,6 @@
 #define HWCAP2_RNG		(1 << 16)
 #define HWCAP2_BTI		(1 << 17)
 #define HWCAP2_MTE		(1 << 18)
+#define HWCAP2_ECV		(1 << 19)
 
 #endif /* _UAPI__ASM_HWCAP_H */

arch/arm64/kernel/cpufeature.c

@@ -279,7 +279,7 @@ static const struct arm64_ftr_bits ftr_id_aa64zfr0[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64mmfr0[] = {
-	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_ECV_SHIFT, 4, 0),
+	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_ECV_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_FGT_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR0_EXS_SHIFT, 4, 0),
 	/*
@@ -2461,6 +2461,7 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 #ifdef CONFIG_ARM64_MTE
 	HWCAP_CAP(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_MTE_SHIFT, FTR_UNSIGNED, ID_AA64PFR1_MTE, CAP_HWCAP, KERNEL_HWCAP_MTE),
 #endif /* CONFIG_ARM64_MTE */
+	HWCAP_CAP(SYS_ID_AA64MMFR0_EL1, ID_AA64MMFR0_ECV_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, KERNEL_HWCAP_ECV),
 	{},
 };
 

arch/arm64/kernel/cpuinfo.c

@@ -94,6 +94,7 @@ static const char *const hwcap_str[] = {
 	[KERNEL_HWCAP_RNG]		= "rng",
 	[KERNEL_HWCAP_BTI]		= "bti",
 	[KERNEL_HWCAP_MTE]		= "mte",
+	[KERNEL_HWCAP_ECV]		= "ecv",
 };
 
 #ifdef CONFIG_COMPAT