Hi Valentin,

On Friday 24 Jan 2020 at 12:00:25 (+0000), Valentin Schneider wrote:

On 23/01/2020 18:32, Ionela Voinescu wrote:
[...]

quoted

and later we can use information in
AMCGCR_EL0 to get the number of architected counters (n) and
AMEVTYPER0<n>_EL0 to find out the type. The same logic would apply to
the auxiliary counters.

Good, I think that's all we'll really need. I've not gone through the whole
series (yet!) so I might've missed AMCGCR being used.

No, it's not used later in the patches either, specifically because
this is version 1 and we should be able to rely on these first 4
architected counters for all future versions of the AMU implementation.

quoted

quoted

quoted

@@ -1150,6 +1152,59 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
 
 #endif
 
+#ifdef CONFIG_ARM64_AMU_EXTN
+
+/*
+ * This per cpu variable only signals that the CPU implementation supports
+ * the Activity Monitors Unit (AMU) but does not provide information
+ * regarding all the events that it supports.
+ * When this amu_feat per CPU variable is true, the user of this feature
+ * can only rely on the presence of the 4 fixed counters. But this does
+ * not guarantee that the counters are enabled or access to these counters
+ * is provided by code executed at higher exception levels.
+ *
+ * Also, to ensure the safe use of this per_cpu variable, the following
+ * accessor is defined to allow a read of amu_feat for the current cpu only
+ * from the current cpu.
+ *  - cpu_has_amu_feat()
+ */
+static DEFINE_PER_CPU_READ_MOSTLY(u8, amu_feat);
+

Why not bool?

I've changed it from bool after a sparse warning about expression using
sizeof(bool) and found this is due to sizeof(bool) being compiler
dependent. It does not change anything but I thought it might be a good
idea to define it as 8-bit unsigned and rely on fixed size.

I believe conveying the intent (a truth value) is more important than the
underlying storage size in this case. It mostly matters when dealing with
aggregates, but here it's just a free-standing variable.

We already have a few per-CPU boolean variables in arm64/kernel/fpsimd.c
and the commits aren't even a year old, so I'd go for ignoring sparse this
time around.

Will do!

Thanks,
Ionela.

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