-----Original Message-----
From: Wood Scott-B07421
Sent: Wednesday, February 13, 2013 6:53 AM
To: Bhushan Bharat-R65777
Cc: Wood Scott-B07421; Michael Neuling; linuxppc-dev@lists.ozlabs.org
Subject: Re: BOOKE KVM calling load_up_fpu from C?
=20
On 02/12/2013 07:18:14 PM, Bhushan Bharat-R65777 wrote:

quoted

quoted

-----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

quoted

Subject: Re: BOOKE KVM calling load_up_fpu from C?

On 02/12/2013 03:01:07 AM, Bhushan Bharat-R65777 wrote:

quoted

To me this looks wrong. And this seems to works because the
thread->reg->msr is not actually used to write SRR1 (and

eventually

quoted

quoted

the thread MSR) when doing rfi to enter guest. Infact
Guest(shadow_msr) MSR is used as SRR1 and which will have proper

MSR

quoted

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(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

quoted

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 ?

=20
What epilog?  We're talking about the case where it's called from C code
=20
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.

Agree. Was just confirming the exception handler case.

=20

quoted

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 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 ?

=20
No, r12 is a volatile register in the ABI, as is r9.  The issue is that t=

he

stack can be corrupted.

Ok, Thanks
-Bharat

=20
-Scott