[Qemu-devel] TBL register permissions for PPC

[Dima Ilyevsky <ilyevsky@gmail.com>]

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Hello All,

I have a question about read permissions of TBL SPR for all ppc processors:
I have discovered that my application, compiled by WindRiver diab compiler
and running in vxworks OS on ppc405 architecture bumps into exception
generated when trying to read TBL or TBU registers:


program
Exception current instruction address: 0x0003eb28
Machine Status Register: 0x00088200
Condition Register: 0x40000440

<<<<<Registers>>>>>

.....

<<<<<Disassembly>>>>>

 0x3eb08  4e800020    blr
 0x3eb0c  3860ffff    li          r3,0xffff # -1
 0x3eb10  4e800020    blr
 0x3eb14  38a00000    li          r5,0x0 # 0
 0x3eb18  7cbc43a6    mtspr       TBL,r5
 0x3eb1c  7c7d43a6    mtspr       TBU,r3
 0x3eb20  7c9c43a6    mtspr       TBL,r4
 0x3eb24  4e800020    blr
*0x3eb28  7cbd42a6    mfspr       r5,TBU
 0x3eb2c  7cdc42a6    mfspr       r6,TBL
 0x3eb30  7cfd42a6     mfspr       r7,TBU
 0x3eb34  7c053800    cmp         crf0,0,r5,r7
 0x3eb38  4082fff0       bc          0x4,2, 0x3eb28 # vxTimeBaseGet
 0x3eb3c  90a30000    stw         r5,0(r3)
 0x3eb40  90c40000    stw         r6,0(r4)
 0x3eb44  4e800020    blr

<<<<<Traceback>>>>>
......


I looked into qemu mfspr instruction implementation and discovered that for
some reason qemu had set NOACCESS permissions for reading this register upon
CPU initialization:

/* Generic PowerPC time base */
static void gen_tbl (CPUPPCState *env)
{
    spr_register(env, SPR_VTBL,  "TBL",
                 &spr_read_tbl, SPR_NOACCESS,
                 &spr_read_tbl, SPR_NOACCESS,
                 0x00000000);
    spr_register(env, SPR_TBL,   "TBL",
                 *SPR_NOACCESS*, SPR_NOACCESS,
                 *SPR_NOACCESS*, &spr_write_tbl,
                 0x00000000);
    spr_register(env, SPR_VTBU,  "TBU",
                 &spr_read_tbu, SPR_NOACCESS,
                 &spr_read_tbu, SPR_NOACCESS,
                 0x00000000);
    spr_register(env, SPR_TBU,   "TBU",
                 *SPR_NOACCESS*, SPR_NOACCESS,
                 *SPR_NOACCESS*, &spr_write_tbu,
                 0x00000000);
}


Is this a right thing to do? it's certainly either qemu bug or diab bug
(which i don't think is likely, cause this code has been running on the real
hw without any issues)


BR,
Dmitry I.
-- 


Jonathan Swift<http://www.brainyquote.com/quotes/authors/j/jonathan_swift.html>
- "May you live every day of your life."

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