On 02/12/2013 07:18:14 PM, Bhushan Bharat-R65777 wrote:
> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Wednesday, February 13, 2013 12:03 AM
> To: Bhushan Bharat-R65777
> Cc: Michael Neuling; Wood Scott-B07421;
linuxppc-dev@lists.ozlabs.org
> Subject: Re: BOOKE KVM calling load_up_fpu from C?
>
> On 02/12/2013 03:01:07 AM, Bhushan Bharat-R65777 wrote:
> > To me this looks wrong. And this seems to works because the
> > thread->reg->msr is not actually used to write SRR1 (and
eventually
> > the thread MSR) when doing rfi to enter guest. Infact
> > Guest(shadow_msr) MSR is used as SRR1 and which will have proper
MSR
> > (including FP set).
> >
> > But Yes, Scott is right person to comment, So let us wait for him
> > comment.
>
> I don't think it's actually a problem on 32-bit, since r9 is
modified but never
> actually used for anything.
Is not the epilog loads srr1 in r9 and load_up_fpu() changes r9 and
then r9 is written back in srr1 ?
What epilog? We're talking about the case where it's called from C
code.
When it's called from an exception handler, then r9 is used, but in
that case it's also initialized before calling load_up_fpu, by the
prolog.
> On 64-bit, though, there's a store to the caller's
> stack frame (yuck) which the kvm/booke.h caller is not prepared for.
So if caller is using r12 then it can lead to come corruption, right ?
No, r12 is a volatile register in the ABI, as is r9. The issue is that
the stack can be corrupted.
-Scott
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