On 02/12/2013 03:01:07 AM, Bhushan Bharat-R65777 wrote:
> -----Original Message-----
> From: Michael Neuling [mailto:mi...@neuling.org]
> Sent: Tuesday, February 12, 2013 9:46 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 <r65...@freescale.com> wrote:
>
> >
> >
> > > -----Original Message-----
> > > From: Michael Neuling [mailto:mi...@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?
> > >
> > > 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.
I don't think it's actually a problem on 32-bit, since r9 is modified
but never actually used for anything. 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. Indeed, book3s's kvmppc_load_up_fpu creates an
interrupt-like stack frame, but does not load r9 or r12.
It would be really nice if assumptions like these were put in a code
comment above load_up_fpu... and if we didn't have so many random
differences between 32-bit and 64-bit. :-P
-Scott
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