[Qemu-devel] [MIPS Malta] Wrong relative jump in the YAMON print subroutine

[Jakub Jermar <jakub@jermar.eu>]

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Hello,

there appears to be a bug in the hand-written machine code which causes
the YAMON print subroutine to jump to a wrong location after printing
the first character. In hw/mips_malta.c, line 619, there is:

stl_raw(p++, 0x08000205);                                    /* j 814 */

which results in the following wrong code being generated:

   0xbfc00814:	lbu	a0,0(t2)
   0xbfc00818:	addiu	t2,t2,1
   0xbfc0081c:	beqz	a0,0xbfc00834
   0xbfc00820:	nop
   0xbfc00824:	jal	0xbfc00870
   0xbfc00828:	nop
   0xbfc0082c:	j	0xb0000814     <==== HERE

While in fact we would like it to branch to 0xbfc00814 instead. To
achieve this effect, the line needs to be changed to read as:

stl_raw(p++, 0x1000fff9);                                   /* j 814 */

so that we get:

   0xbfc00814:	lbu	a0,0(t2)
   0xbfc00818:	addiu	t2,t2,1
   0xbfc0081c:	beqz	a0,0xbfc00834
   0xbfc00820:	nop
   0xbfc00824:	jal	0xbfc00870
   0xbfc00828:	nop
   0xbfc0082c:	b	0xbfc00814

I verified the print subroutine works as expected with the fix.

Please find the fix attached to this message.

Regards,
Jakub

[-- Attachment #2: yamon-print.patch --]
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diff --git a/hw/mips_malta.c b/hw/mips_malta.c
index dfd7b6b..77a8e88 100644
--- a/hw/mips_malta.c
+++ b/hw/mips_malta.c
@@ -616,7 +616,7 @@ static void write_bootloader (CPUMIPSState *env, uint8_t *base,
     stl_raw(p++, 0x00000000);                                     /* nop */
     stl_raw(p++, 0x0ff0021c);                                     /* jal 870 */
     stl_raw(p++, 0x00000000);                                     /* nop */
-    stl_raw(p++, 0x08000205);                                     /* j 814 */
+    stl_raw(p++, 0x1000fff9);                                     /* j 814 */
     stl_raw(p++, 0x00000000);                                     /* nop */
     stl_raw(p++, 0x01a00008);                                     /* jr t5 */
     stl_raw(p++, 0x01602021);                                     /* move a0,t3 */