arm64/sve: Document that __SVE_VQ_MAX is much larger than needed

__SVE_VQ_MAX is defined without comment as 512 but the actual
architectural maximum is 16, a substantial difference which might not
be obvious to readers especially given the several different units used
for specifying vector sizes in various contexts and the fact that it's
often used via macros.  In an effort to minimise surprises for users who
might assume the value is the architectural maximum and use it to do
things like size allocations add a comment noting the difference, and
add a note for SVE_VQ_MAX to aid discoverability.
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/20240209-arm64-sve-vl-max-comment-v2-1-111b283469ee@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/uapi/asm/sve_context.h

@@ -13,6 +13,17 @@
 
 #define __SVE_VQ_BYTES		16	/* number of bytes per quadword */
 
+/*
+ * Yes, __SVE_VQ_MAX is 512 QUADWORDS.
+ *
+ * To help ensure forward portability, this is much larger than the
+ * current maximum value defined by the SVE architecture.  While arrays
+ * or static allocations can be sized based on this value, watch out!
+ * It will waste a surprisingly large amount of memory.
+ *
+ * Dynamic sizing based on the actual runtime vector length is likely to
+ * be preferable for most purposes.
+ */
 #define __SVE_VQ_MIN		1
 #define __SVE_VQ_MAX		512