aarch64: Document _SC_LEVEL1_DCACHE_LINESIZE caveat

The _SC_LEVEL1_DCACHE_LINESIZE is reported using the contents of the
ctr_el0 register, which tells us the minimum observable cache line
size by userspace.  This typically is the same as the L1 cache line
size, but that may not always be true.  It could be a higher level
cache line size as long as cache cleaning and invalidation work
correctly with that line size in userspace.  The falkor core for
example reports the L2 line size as the dcache line size in CTR_EL0
while also reporting the correct L1 dcache line size via CCSIDR_EL1.

	* manual/conf.texi (_SC_LEVEL1_DCACHE_LINESIZE,
	_SC_LEVEL1_ICACHE_LINESIZE): Document aarch64 caveat.

Reviewed-by: Rical Jasan <ricaljasan@pacific.net>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/manual/conf.texi b/manual/conf.texi
index 079bdb295e..62ab444139 100644
--- a/manual/conf.texi
+++ b/manual/conf.texi
@@ -690,6 +690,12 @@ Inquire about the associativity of the Level 1 instruction cache.
 @standards{GNU, unistd.h}
 Inquire about the line length of the Level 1 instruction cache.
 
+On aarch64, the cache line size returned is the minimum instruction cache line
+size observable by userspace.  This is typically the same as the L1 icache
+size but on some cores it may not be so.  However, it is specified in the
+architecture that operations such as cache line invalidation are consistent
+with the size reported with this variable.
+
 @item _SC_LEVEL1_DCACHE_SIZE
 @standards{GNU, unistd.h}
 Inquire about the size of the Level 1 data cache.
@@ -702,6 +708,12 @@ Inquire about the associativity of the Level 1 data cache.
 @standards{GNU, unistd.h}
 Inquire about the line length of the Level 1 data cache.
 
+On aarch64, the cache line size returned is the minimum data cache line size
+observable by userspace.  This is typically the same as the L1 dcache size but
+on some cores it may not be so.  However, it is specified in the architecture
+that operations such as cache line invalidation are consistent with the size
+reported with this variable.
+
 @item _SC_LEVEL2_CACHE_SIZE
 @standards{GNU, unistd.h}
 Inquire about the size of the Level 2 cache.