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From: Michael Neuling <mikey@neuling.org>
To: Bhushan Bharat-R65777 <R65777@freescale.com>
Subject: Re: BOOKE KVM calling load_up_fpu from C?
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 <6A3DF150A5B70D4F9B66A25E3F7C888D0659EC82@039-SN2MPN1-023.039d.mgd.msft.net>
Date: Tue, 12 Feb 2013 15:16:18 +1100
Message-ID: <30014.1360642578@ale.ozlabs.ibm.com>
Cc: Wood Scott-B07421 <B07421@freescale.com>,
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Bhushan Bharat-R65777 <R65777@freescale.com> wrote:

> 
> 
> > -----Original Message-----
> > From: Michael Neuling [mailto:mikey@neuling.org]
> > Sent: Tuesday, February 12, 2013 9:16 AM
> > To: Bhushan Bharat-R65777
> > Cc: Wood Scott-B07421; linuxppc-dev@lists.ozlabs.org
> > Subject: Re: BOOKE KVM calling load_up_fpu from C?
> > 
> > Bhushan Bharat-R65777 <R65777@freescale.com> wrote:
> > 
> > >
> > >
> > > > -----Original Message-----
> > > > From: Linuxppc-dev [mailto:linuxppc-dev-
> > > > bounces+bharat.bhushan=freescale.com@lists.ozlabs.org] On Behalf Of
> > > > bounces+Michael
> > > > Neuling
> > > > Sent: Tuesday, February 12, 2013 8:59 AM
> > > > To: Wood Scott-B07421
> > > > Cc: linuxppc-dev@lists.ozlabs.org
> > > > Subject: BOOKE KVM calling load_up_fpu from C?
> > > >
> > > > Scott,
> > > >
> > > > I was looking at changing how load_up_fpu works and I found this in
> > > > arch/powerpc/kvm/booke.h:
> > > >
> > > > static inline void kvmppc_load_guest_fp(struct kvm_vcpu *vcpu) {
> > > > #ifdef CONFIG_PPC_FPU
> > > > 	if (vcpu->fpu_active && !(current->thread.regs->msr & MSR_FP)) {
> > > > 		load_up_fpu();
> > > > 		current->thread.regs->msr |= MSR_FP;
> > > > 	}
> > > > #endif
> > > > }
> > > >
> > > > I'm wondering how this is suppose to work since load_up_fpu is
> > > > suppose to have MSR in R12?
> > >
> > > Is not the load_up_fpu() does mfmsr:
> > >
> > > _GLOBAL(load_up_fpu)
> > >         mfmsr   r5
> > >         ori     r5,r5,MSR_FP
> > > #ifdef CONFIG_VSX
> > > BEGIN_FTR_SECTION
> > >         oris    r5,r5,MSR_VSX@h
> > > END_FTR_SECTION_IFSET(CPU_FTR_VSX)
> > > #endif
> > >         SYNC
> > >         MTMSRD(r5)                      /* enable use of fpu now */
> > >         isync
> > > <snip>
> > 
> > Look further down...
> > 
> > #ifdef CONFIG_PPC32
> > 	mfspr	r5,SPRN_SPRG_THREAD		/* current task's THREAD (phys) */
> > 	lwz	r4,THREAD_FPEXC_MODE(r5)
> > 	ori	r9,r9,MSR_FP		/* enable FP for current */
> > 	or	r9,r9,r4
> > #else
> > 	ld	r4,PACACURRENT(r13)
> > 	addi	r5,r4,THREAD		/* Get THREAD */
> > 	lwz	r4,THREAD_FPEXC_MODE(r5)
> > 	ori	r12,r12,MSR_FP
> > 	or	r12,r12,r4
> > 	std	r12,_MSR(r1)
> > #endif
> > 
> > R12 is loaded with SRR1 in the exception prolog before load_up_fpu is called.
> 
> Yes it is SRR1 not MSR.

Yes, SRR1 == the MSR of the user process, not the current MSR.

> Also on 32bit it looks like that R9 is assumed to have SRR1.

Yep that too.  

So any idea how it's suppose to work or is it broken?

Mikey