On Wed, Dec 09, 2020 at 06:35:09PM +0000, Catalin Marinas wrote:

On Wed, Dec 09, 2020 at 04:39:50PM +0000, Will Deacon wrote:

quoted

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index da250e4741bd..3424f5881390 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h

@@ -764,6 +764,18 @@ static inline bool cpu_has_hw_af(void)
 						ID_AA64MMFR1_HADBS_SHIFT);
 }
 
+static inline bool system_has_hw_af(void)
+{
+	u64 mmfr1;
+
+	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
+		return false;
+
+	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
+	return cpuid_feature_extract_unsigned_field(mmfr1,
+						ID_AA64MMFR1_HADBS_SHIFT);
+}

Could we not add a new system-wide cpu feature that checks for hardware
AF? This read_sanitised_ftr_reg() does a binary search on each
invocation.

I posted a diff [1] which would allow removing the binary search for cases
where we can pass the register coding as a constant (like this), but
honestly, it's not like we have many ID registers so I doubt it really
matters in the grand scheme of things.

That said, I'm spinning a v2 anyway so I can include it for comments
since I haven't posted it as a proper patch before.

Will

[1] https://lore.kernel.org/r/20201202172727.GC29813@willie-the-truck (local)

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