[RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support

[Yury Norov]

This patch is on top of https://lkml.org/lkml/2015/9/2/413

In master, there's only a single function -
	update_mixed_endian_el0_support
And similar function is on review mentioned above.

The algorithm for them is like this:
 - there's system-wide boolean marker for the feature that is
   initially enabled;
 - there's also updater for the feature that may disable it
   system-widely if feature is not supported on current CPU.
 - updater is called for each CPU on bootup.

The problem is the way updater does its work. On each CPU, it
unconditionally updates system-wide marker. For multi-core
system it makes CPU issue invalidate message for a cache
line containing marker. This invalidate increases cache
contention for nothing, because there's a single marker reset
that is really needed, and the others are useless.

If the number of system-wide markers of this sort will grow,
it may become a trouble on large-scale SOCs. The fix is trivial,
though: do system-wide marker update conditionally, and preserve
corresponding cache line in shared state for all update() calls,
except, probably, one.

Signed-off-by: Yury Norov <ynorov at caviumnetworks.com>
---
 arch/arm64/kernel/cpuinfo.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index 4a6ae31..9972c1e 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -87,12 +87,14 @@ bool system_supports_aarch32_el0(void)
 
 static void update_mixed_endian_el0_support(struct cpuinfo_arm64 *info)
 {
-	mixed_endian_el0 &= id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0);
+	if (mixed_endian_el0 && !id_aa64mmfr0_mixed_endian_el0(info->reg_id_aa64mmfr0))
+		mixed_endian_el0 = false;
 }
 
 static void update_aarch32_el0_support(struct cpuinfo_arm64 *info)
 {
-	aarch32_el0 &= id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0);
+	if (aarch32_el0 && !id_aa64pfr0_aarch32_el0(info->reg_id_aa64pfr0))
+		aarch32_el0 = false;
 }
 
 static void update_cpu_features(struct cpuinfo_arm64 *info)
-- 
2.1.4