Re: [Qemu-devel] [PATCH] target-ppc: clear MSR_POW on interrupt

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Fri, Sep 10, 2010 at 05:50:27PM +0200, Alexander Graf wrote:
> Thomas Monjalon wrote:
> > Alexander Graf wrote:
> >   
> >> Thomas Monjalon wrote:
> >>     
> >>> Alexander Graf wrote:
> >>>       
> >>>> Thomas Monjalon wrote:
> >>>>         
> >>>>> From: till <608107@bugs.launchpad.net>
> >>>>>
> >>>>> According to FreeScale's 'Programming Environments Manual for 32-bit
> >>>>> Implementations of the PowerPC Architecture' [MPCFPE32B, Rev.3,
> >>>>> 9/2005], section 6.5, table 6-7, an interrupt resets MSR_POW to zero
> >>>>> but qemu-0.12.4 fails to do so.
> >>>>> Resetting the bit is necessary in order to bring the processor out of
> >>>>> power management since otherwise it goes to sleep right away in the
> >>>>> exception handler, i.e., it is impossible to leave PM-mode.
> >>>>>           
> >>>> This doesn't look right. POW shouldn't even get stored in SRR1. Could
> >>>> you please redo the patch and make sure that mtmsr masks out MSR_POW?
> >>>>         
> >>> Could you point sections of the specification for these requirements ?
> >>>
> >>> I think SRR1 can save any bits. Non significant bits can be overriden and
> >>> are masked out when RFI occurs.
> >>>       
> >> I'm not saying I'm 100% sure on this, but take a look at the e300
> >> reference manual
> >> (http://cache.freescale.com/files/32bit/doc/ref_manual/e300coreRM.pdf)
> >> section 5.2.3. POW is bit 13 in this notion. I don't see it getting
> >> saved to SRR1.
> >>     
> >
> > The current implementation (commit c3d420e) save all the bits and restore only 
> > the needed ones (excluding POW). So POW is cleared after an interrupt.
> >
> > But the subject of this patch wasn't on saved bits. It is to clear POW in the 
> > interrupt context. For an unknown reason, it's not done and it's clearly a bug 
> > to fix.
> >   
> 
> I completely agree. In fact, the mere fact that a bit can slip through
> this loop indicates that something goes wrong.
> 
> As far as SRR1 goes, PowerISA 2.06 draws an even more complicated
> picture: Page 816 says that SRR1 bits 42:44 indicate the power save exit
> reason while 46:47 indicate how much state was transferred. Bit 45 would
> be MSR_POW which is defined as "set to 0".
> 
> Because that means that we'll have to set MSR_POW in to 0 in SRR1 and
> the interrupt switching function, we can just set it to 0 on mtmsr. That
> was my point :).

Hi,

I'm not very familiar with PPC but the way I interpret it, we should clear
the POW bit in both MSR and SRR1 when the interrupt waking the CPU is
taken. While the CPU is sleeping, the POW bit should remain set in MSR
though.

As Alex comments, the submited patch fails to make sure the SRR1 bit
13 is cleared when taking the interrupt, but I don't think clearing
MSR[POW] in mtmsr is the right thing. In practice it probably doesn't
matter much, possibly a bit annoying if you inspect the sleeping state
from a debugger or similar.

I think mtmsr should set MSR[POW] and powerpc_excp() should clear
MSR[POW] prior to copying MSR into SRR1.

Not sure, but thats the way I interpret it...

Cheers