arm64: cpufeature: Add an overview comment for the cpufeature framework

Now that Suzuki isn't within throwing distance, I thought I'd better add
a rough overview comment to cpufeature.c so that it doesn't take me days
to remember how it works next time.
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20200421142922.18950-9-will@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/cpufeature.c

@@ -3,6 +3,56 @@
  * Contains CPU feature definitions
  *
  * Copyright (C) 2015 ARM Ltd.
+ *
+ * A note for the weary kernel hacker: the code here is confusing and hard to
+ * follow! That's partly because it's solving a nasty problem, but also because
+ * there's a little bit of over-abstraction that tends to obscure what's going
+ * on behind a maze of helper functions and macros.
+ *
+ * The basic problem is that hardware folks have started gluing together CPUs
+ * with distinct architectural features; in some cases even creating SoCs where
+ * user-visible instructions are available only on a subset of the available
+ * cores. We try to address this by snapshotting the feature registers of the
+ * boot CPU and comparing these with the feature registers of each secondary
+ * CPU when bringing them up. If there is a mismatch, then we update the
+ * snapshot state to indicate the lowest-common denominator of the feature,
+ * known as the "safe" value. This snapshot state can be queried to view the
+ * "sanitised" value of a feature register.
+ *
+ * The sanitised register values are used to decide which capabilities we
+ * have in the system. These may be in the form of traditional "hwcaps"
+ * advertised to userspace or internal "cpucaps" which are used to configure
+ * things like alternative patching and static keys. While a feature mismatch
+ * may result in a TAINT_CPU_OUT_OF_SPEC kernel taint, a capability mismatch
+ * may prevent a CPU from being onlined at all.
+ *
+ * Some implementation details worth remembering:
+ *
+ * - Mismatched features are *always* sanitised to a "safe" value, which
+ *   usually indicates that the feature is not supported.
+ *
+ * - A mismatched feature marked with FTR_STRICT will cause a "SANITY CHECK"
+ *   warning when onlining an offending CPU and the kernel will be tainted
+ *   with TAINT_CPU_OUT_OF_SPEC.
+ *
+ * - Features marked as FTR_VISIBLE have their sanitised value visible to
+ *   userspace. FTR_VISIBLE features in registers that are only visible
+ *   to EL0 by trapping *must* have a corresponding HWCAP so that late
+ *   onlining of CPUs cannot lead to features disappearing at runtime.
+ *
+ * - A "feature" is typically a 4-bit register field. A "capability" is the
+ *   high-level description derived from the sanitised field value.
+ *
+ * - Read the Arm ARM (DDI 0487F.a) section D13.1.3 ("Principles of the ID
+ *   scheme for fields in ID registers") to understand when feature fields
+ *   may be signed or unsigned (FTR_SIGNED and FTR_UNSIGNED accordingly).
+ *
+ * - KVM exposes its own view of the feature registers to guest operating
+ *   systems regardless of FTR_VISIBLE. This is typically driven from the
+ *   sanitised register values to allow virtual CPUs to be migrated between
+ *   arbitrary physical CPUs, but some features not present on the host are
+ *   also advertised and emulated. Look at sys_reg_descs[] for the gory
+ *   details.
  */
 
 #define pr_fmt(fmt) "CPU features: " fmt