Add comments to vDSO hwcap loading process.

Loading of the vDSO pseudo-hwcap from the type 2 GNU note is
a rather arcane and poorly documented process. Given that I had
a chance to review this code today I thought I would add all
of the things I had to lookup to verify the validity of the
process.

With a single .note.GNU the vDSO can register up to 64 flags,
though in practice you are limited to 64 - _DL_FIRST_EXTRA
bits which on x86 is 12 bits.

The only use of this that I know of is in the Xen support
in Linux where they use the 1st bit to indicate "nosegneg".
I see "We use bit 1 to avoid bugs in some versions of glibc
when bit 0 is used; the choice is otherwise arbitrary.", but
no reference to a glibc bug anywhere. The code as-is should
support bit zero, so we still have that free for future use.

The kernel, glibc, and ld.so.cache must coordinate to ensure
that bit values don't go too high and are used consistently.

---

2013-05-13  Carlos O'Donell  <carlos@redhat.com>

	* elf/dl-hwcaps.c (_dl_important_hwcaps): Comment vDSO hwcap loading.
	* elf/ldconfig.c (is_hwcap_platform): Comment each hwcap check.
	(main): Comment "tls" pseudo-hwcap.

1 files changed, 6 insertions, 0 deletions

diff --git a/elf/ldconfig.c b/elf/ldconfig.c
index 57c6a6f..340c132 100644
--- a/elf/ldconfig.c
+++ b/elf/ldconfig.c
@@ -173,13 +173,17 @@ is_hwcap_platform (const char *name)
 {
   int hwcap_idx = _dl_string_hwcap (name);
 
+  /* Is this a normal hwcap for the machine e.g. fpu?  */
   if (hwcap_idx != -1 && ((1 << hwcap_idx) & hwcap_mask))
     return 1;
 
+  /* ... Or is it a platform pseudo-hwcap e.g. i686?  */
   hwcap_idx = _dl_string_platform (name);
   if (hwcap_idx != -1)
     return 1;
 
+  /* ... Or is this one of the extra pseudo-hwcaps that we map beyond
+     _DL_FIRST_EXTRA e.g. tls, or nosegneg?  */
   for (hwcap_idx = _DL_FIRST_EXTRA; hwcap_idx < 64; ++hwcap_idx)
     if (hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA] != NULL
 	&& !strcmp (name, hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA]))
@@ -1265,6 +1269,8 @@ main (int argc, char **argv)
 	  add_dir (argv[i]);
     }
 
+  /* The last entry in hwcap_extra is reserved for the "tls"
+     pseudo-hwcap which indicates support for TLS.  */
   hwcap_extra[63 - _DL_FIRST_EXTRA] = "tls";
 
   set_hwcap ();