svn commit: r294846 - head/sys/mips/mips

[Warner Losh]

Author: imp
Date: Tue Jan 26 18:39:23 2016
New Revision: 294846
URL: https://svnweb.freebsd.org/changeset/base/294846

Log:
  Add a comment about why at is turned off in the exception handler.
  Only k0 and k1 may be touched until we save registers somewhere.
  
  MFC After: 2 days

Modified:
  head/sys/mips/mips/exception.S

Modified: head/sys/mips/mips/exception.S
==============================================================================
--- head/sys/mips/mips/exception.S	Tue Jan 26 18:32:48 2016	(r294845)
+++ head/sys/mips/mips/exception.S	Tue Jan 26 18:39:23 2016	(r294846)
@@ -177,6 +177,12 @@ MipsDoTLBMiss:
 VECTOR(MipsException, unknown)
 /*
  * Find out what mode we came from and jump to the proper handler.
+ *
+ * Note: at turned off here because we cannot trash the at register
+ * in this exception code. Only k0 and k1 may be modified before
+ * we save registers. This is true of all functions called through
+ * the pointer magic: Mips{User,Kern}Intr, Mips{User,Kern}GenException
+ * and MipsTLBInvalidException
  */
 	.set	noat
 	mfc0	k0, MIPS_COP_0_STATUS		# Get the status register