From c8719df5f911f5c042d61045788f57087e4b46f5 Mon Sep 17 00:00:00 2001 From: Warner Losh Date: Tue, 26 Jan 2016 18:39:23 +0000 Subject: [PATCH] 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 --- sys/mips/mips/exception.S | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/sys/mips/mips/exception.S b/sys/mips/mips/exception.S index 6eddd19af882..01fd2106e53a 100644 --- a/sys/mips/mips/exception.S +++ b/sys/mips/mips/exception.S @@ -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