BOOKE KVM calling load_up_fpu from C?

[Bhushan Bharat-R65777]

> -----Original Message-----
> From: Michael Neuling [mailto:mikey at neuling.org]
> Sent: Tuesday, February 12, 2013 9:46 AM
> To: Bhushan Bharat-R65777
> Cc: Wood Scott-B07421; linuxppc-dev at lists.ozlabs.org
> Subject: Re: BOOKE KVM calling load_up_fpu from C?
> 
> Bhushan Bharat-R65777 <R65777 at freescale.com> wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: Michael Neuling [mailto:mikey at neuling.org]
> > > Sent: Tuesday, February 12, 2013 9:16 AM
> > > To: Bhushan Bharat-R65777
> > > Cc: Wood Scott-B07421; linuxppc-dev at lists.ozlabs.org
> > > Subject: Re: BOOKE KVM calling load_up_fpu from C?
> > >
> > > Bhushan Bharat-R65777 <R65777 at freescale.com> wrote:
> > >
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Linuxppc-dev [mailto:linuxppc-dev-
> > > > > bounces+bharat.bhushan=freescale.com at lists.ozlabs.org] On Behalf
> > > > > bounces+Of Michael
> > > > > Neuling
> > > > > Sent: Tuesday, February 12, 2013 8:59 AM
> > > > > To: Wood Scott-B07421
> > > > > Cc: linuxppc-dev at 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 at 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?

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.

Thanks
-Bharat