Re: FP emulation patch available

["Kevin D. Kissell" <kevink@mips.com>]

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>My DS 5000/133 (R3000A) with FPU disabled and FPU emulation shows:
> Illegal instruction 00000034 at 801ce924, ...
>
>System.map shows:
> 801ce920 b dsemul_insns
> 801ce928 b dsemul_cpc
>
>Looks like your trick in mips_dsemul() doesn't work too well for ISA-I CPUs. Do
>you have an idea for an alternative?


I have come up with a slightly-less-pretty hack that uses the
Load Address Error trap instead of the Trap instruction to force
kernel entry in the delay slot emulator. It seems just as functional
as the previous version (i.e. operational but "paranoia" finds an
exponentiation problem), and is currently being tortured with crashme
to see if it holds up under corrupted instruction streams and corrupted
process states.  I attach a pseudo-patch (cvs diff -c output) for the changes
relative to the version obtained by applying the previous patches on the
paralogos.com server, and would appreicate verification that it does
indeed work on an R3K.  If it does, I'll check it into the MIPS repository
and it will be included in the next web distribution (and maybe our
CD-ROMS).

My apologies to those of you whose mailers can't handle
attachments.

            Regards,

            Kevin K.



[-- Attachment #2: cp1emu.patch --]
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Index: cp1emu.c
===================================================================
RCS file: /home/repository/sw/linux-2.2.12/linux/arch/mips/fpu_emulator/cp1emu.c,v
retrieving revision 1.5
diff -c -r1.5 cp1emu.c
*** cp1emu.c	2000/02/27 15:19:18	1.5
--- cp1emu.c	2000/03/12 12:24:06
***************
*** 767,772 ****
--- 767,774 ----
  static unsigned int	dsemul_sr;
  static void *dsemul_osys;
  
+ #define AdEL 4
+ #define AdELOAD 0x8fa00001
  
  int
  do_dsemulret(struct pt_regs *xcp)
***************
*** 774,781 ****
  #ifdef DSEMUL_TRACE
      _mon_printf ("desemulret\n");
  #endif
!     /* Restore previous Trap instruction vector */
!     (void)set_except_vector(13, dsemul_osys);
      /* Set EPC to return to post-branch instruction */
      xcp->cp0_epc = VA_TO_REG (dsemul_cpc);
      /* 
--- 776,783 ----
  #ifdef DSEMUL_TRACE
      _mon_printf ("desemulret\n");
  #endif
!     /* Restore previous exception vector */
!     (void)set_except_vector(AdEL, dsemul_osys);
      /* Set EPC to return to post-branch instruction */
      xcp->cp0_epc = VA_TO_REG (dsemul_cpc);
      /* 
***************
*** 811,833 ****
       */
      dsemul_insns = (mips_instruction *)(xcp->regs[29] & ~3);
      dsemul_insns -= 3; /* Two instructions, plus one for luck ;-) */
!     /* Verify that space exists, or can be grown, on the stack */
      if(verify_area(VERIFY_WRITE, dsemul_insns, sizeof(mips_instruction)*2)) 
  	return SIGBUS;
      
      dsemul_insns[0] = ir;
      /* 
       * Algorithmics used a system call instruction, and
!      * borrowed that vector.  It seems more prudent, and
!      * is simpler in Linux, to use a TEQ instruction, though
!      * this does require a MIPS II CPU.
       */
- #define TEQ_R0_R0 0x00000034 
-     dsemul_insns[1] = TEQ_R0_R0;
  
      dsemul_cpc = cpc;
      dsemul_sr = xcp->cp0_status;
!     dsemul_osys = set_except_vector(13, handle_dsemulret);
   
      xcp->cp0_epc = VA_TO_REG &dsemul_insns[0];
      xcp->cp0_status &= ~ST0_IM;	/* interrupt disabled inside dsemul! */
--- 813,841 ----
       */
      dsemul_insns = (mips_instruction *)(xcp->regs[29] & ~3);
      dsemul_insns -= 3; /* Two instructions, plus one for luck ;-) */
!     /* Verify that the stack pointer is not competely insane */
      if(verify_area(VERIFY_WRITE, dsemul_insns, sizeof(mips_instruction)*2)) 
  	return SIGBUS;
      
      dsemul_insns[0] = ir;
      /* 
       * Algorithmics used a system call instruction, and
!      * borrowed that vector.  As that would be catastrophic
!      * if a reschedule happens, a TEQ instruction was used
!      * in early versions of the Linux kernel emulator, since 
!      * Linux does nothing useful with Trap instructions.
!      * That does not work on R3000s, however, so here we
!      * steal the Address Error on Load vector and
!      * generate an address error on an unaligned load.
       */
  
+     /* If one is *really* paranoid, one tests for a bad stack pointer */
+     if((xcp->regs[29] & 0x3) == 0x3) dsemul_insns[1] = AdELOAD - 1;
+     else dsemul_insns[1] = AdELOAD;
+ 
      dsemul_cpc = cpc;
      dsemul_sr = xcp->cp0_status;
!     dsemul_osys = set_except_vector(AdEL, handle_dsemulret);
   
      xcp->cp0_epc = VA_TO_REG &dsemul_insns[0];
      xcp->cp0_status &= ~ST0_IM;	/* interrupt disabled inside dsemul! */