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Date: Wed, 2 Dec 2009 02:35:30 +0300 (MSK)
From: malc <av1474@comtv.ru>
Subject: Re: [Qemu-devel] TBL register permissions for PPC
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To: Alexander Graf <agraf@suse.de>
Cc: Dima Ilyevsky <ilyevsky@gmail.com>, qemu-devel@nongnu.org

On Wed, 2 Dec 2009, Alexander Graf wrote:

> 
> 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);
> }
> 
> agraf@lychee:/tmp> ./mftb 
> TB: 0xc0397180 0x603
> 
> However it can't be written to:
> 
> asm("mttbl %0" : : "r" (tbl));
> 
> agraf@lychee:/tmp> ./mftb 
> Illegal instruction


7.2.1 Writing the Time Base
Writing the Time Base is privileged, and can be done
only in hypervisor state. Reading the Time Base is not
privileged; it is discussed in Chapter 4 of Book II.

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

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