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Re: [Qemu-devel] TBL register permissions for PPC


From: Alexander Graf
Subject: Re: [Qemu-devel] TBL register permissions for PPC
Date: Wed, 2 Dec 2009 00:23:41 +0100

On 01.12.2009, at 19:33, Dima Ilyevsky wrote:

> 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:

Unless Linux does something funky, mftlb, mftbu (and mftb on 64 bit) are 
readable from PR=1.

int main()
{
    long tbu=0, tbl=0;

    asm("mftbu %0" : "=r" (tbu));
    asm("mftbl %0" : "=r" (tbl));

    printf("TB: %#x %#x\n", tbl, tbu);
}

address@hidden:/tmp> ./mftb 
TB: 0xc0397180 0x603

However it can't be written to:

asm("mttbl %0" : : "r" (tbl));

address@hidden:/tmp> ./mftb 
Illegal instruction


So yes, I'd suspect a bug in qemu here. Feel free to send a patch.

Alex



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