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Re: [Qemu-devel] CP0 after reset bug (Was: Add MIPS ELF loader)

From: Alexander Voropay
Subject: Re: [Qemu-devel] CP0 after reset bug (Was: Add MIPS ELF loader)
Date: Fri, 21 Apr 2006 16:35:59 +0400

"Alexander Voropay" <address@hidden> wrote:

Another issue:

0xbfc00424:  mtc0       zero,$13
0x0001: raise_exception 0x11

The problem is a code *before* this :
       mfc0    v0,C0_SR
       and     v0,SR_SR                # preserve Soft Reset
       or      v0,SR_BEV               # set Boot Exceptions

       mtc0    v0,C0_SR                # 32 bit, kernel mode, bootstrap
       mtc0    zero,C0_CAUSE     # <-- TRAP there !!!

This code is a cut'n'paste from the "See MIPS Run" p.338

Unfortunately, this code clears CU0  bits in the CP0(SR).
It makes CP0 unusable for program and causes an exception 11 :
Coprocessor Unusable on the next CP0 access.

The Qemu has a bug there. The "See MIPS Run" p.51 states:

CU0 - Coprocessor 0 usable; Set 1 to be able to use some nominally
priveleged instructions in the user mode. You don't want to do this.
The CPU control instructions encoded as coprocessor 0 type are
always usable in kernel mode, regardless of the setting of this bit.

Qemu does simply check:
   if (!(ctx->CP0_Status & (1 << CP0St_CU0)) &&
       !(ctx->hflags & MIPS_HFLAG_UM) &&
       !(ctx->hflags & MIPS_HFLAG_ERL) &&
       !(ctx->hflags & MIPS_HFLAG_EXL)) {
       if (loglevel & CPU_LOG_TB_IN_ASM) {
           fprintf(logfile, "CP0 is not usable\n");
       generate_exception_err (ctx, EXCP_CpU, 0);

This check is not enought to emulate a Coprocessor Unusable
situation on Reset (when CPU is in the kernel mode).


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