kasan: docs: update overview section

Update the "Overview" section in KASAN documentation:

 - Outline main use cases for each mode.

 - Mention that HW_TAGS mode need compiler support too.

 - Move the part about SLUB/SLAB support from "Usage" to "Overview".

 - Punctuation, readability, and other minor clean-ups.

Link: https://lkml.kernel.org/r/1486fba8514de3d7db2f47df2192db59228b0a7b.1615559068.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -11,17 +11,31 @@ designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
 2. software tag-based KASAN (similar to userspace HWASan),
 3. hardware tag-based KASAN (based on hardware memory tagging).
 
-Software KASAN modes (1 and 2) use compile-time instrumentation to insert
+Generic KASAN is mainly used for debugging due to a large memory overhead.
-validity checks before every memory access, and therefore require a compiler
+Software tag-based KASAN can be used for dogfood testing as it has a lower
+memory overhead that allows using it with real workloads. Hardware tag-based
+KASAN comes with low memory and performance overheads and, therefore, can be
+used in production. Either as an in-field memory bug detector or as a security
+mitigation.
+
+Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
+validity checks before every memory access and, therefore, require a compiler
 version that supports that.
 
-Generic KASAN is supported in both GCC and Clang. With GCC it requires version
+Generic KASAN is supported in GCC and Clang. With GCC, it requires version
 8.3.0 or later. Any supported Clang version is compatible, but detection of
 out-of-bounds accesses for global variables is only supported since Clang 11.
 
-Tag-based KASAN is only supported in Clang.
+Software tag-based KASAN mode is only supported in Clang.
 
-Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
+The hardware KASAN mode (#3) relies on hardware to perform the checks but
+still requires a compiler version that supports memory tagging instructions.
+This mode is supported in GCC 10+ and Clang 11+.
+
+Both software KASAN modes work with SLUB and SLAB memory allocators,
+while the hardware tag-based KASAN currently only supports SLUB.
+
+Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
 
 Usage
@@ -39,9 +53,6 @@ For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
 CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
 The former produces smaller binary while the latter is 1.1 - 2 times faster.
 
-Both software KASAN modes work with both SLUB and SLAB memory allocators,
-while the hardware tag-based KASAN currently only support SLUB.
-
 For better error reports that include stack traces, enable CONFIG_STACKTRACE.
 
 To augment reports with last allocation and freeing stack of the physical page,