On 06/05/2013 04:14:21 AM, Caraman Mihai Claudiu-B02008 wrote:
> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Wednesday, June 05, 2013 1:54 AM
> To: Caraman Mihai Claudiu-B02008
> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linuxppc-
> dev@xxxxxxxxxxxxxxxx; Caraman Mihai Claudiu-B02008
> Subject: Re: [RFC PATCH 6/6] KVM: PPC: Book3E: Enhance FPU laziness
>
> On 06/03/2013 03:54:28 PM, Mihai Caraman wrote:
> > Adopt AltiVec approach to increase laziness by calling
> > kvmppc_load_guest_fp()
> > just before returning to guest instaed of each sched in.
> >
> > Signed-off-by: Mihai Caraman <mihai.caraman@xxxxxxxxxxxxx>
>
> If you did this *before* adding Altivec it would have saved a
question
> in an earlier patch. :-)
I kept asking myself about the order and in the end I decided that
this is
an improvement originated from AltiVec work. FPU may be further
cleaned up
(get rid of active state, etc).
>
> > ---
> > arch/powerpc/kvm/booke.c | 1 +
> > arch/powerpc/kvm/e500mc.c | 2 --
> > 2 files changed, 1 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
> > index 019496d..5382238 100644
> > --- a/arch/powerpc/kvm/booke.c
> > +++ b/arch/powerpc/kvm/booke.c
> > @@ -1258,6 +1258,7 @@ int kvmppc_handle_exit(struct kvm_run *run,
> > struct kvm_vcpu *vcpu,
> > } else {
> > kvmppc_lazy_ee_enable();
> > kvmppc_load_guest_altivec(vcpu);
> > + kvmppc_load_guest_fp(vcpu);
> > }
> > }
> >
>
> You should probably do these before kvmppc_lazy_ee_enable().
Why? I wanted to look like part of lightweight_exit.
We want to minimize the portion of the code that runs with interrupts
disabled while telling tracers that interrupts are enabled. We want to
minimize the C code run with lazy EE in an inconsistent state.
The same applies to kvm_vcpu_run()...
> Actually, I don't think this is a good idea at all. As I understand
> it, you're not supposed to take kernel ownersship of floating point
in
> non-atomic context, because an interrupt could itself call
> enable_kernel_fp().
So lightweight_exit isn't executed in atomic context?
Ignore this, I misread what the patch was doing. I thought you were
doing the opposite you did. :-P
As such, this patch appears to fix the thing I was complaining about --
before, we could have taken an interrupt after kvmppc_core_vcpu_load(),
and that interrupt could have claimed the floating point (unlikely with
the kernel as is, but you never know what could happen in the future or
out-of-tree...).
Will be lazyee fixes including kvmppc_fix_ee_before_entry() in 3.10?
64-bit Book3E KVM is unreliable without them. Should we disable e5500
too
for 3.10?
I hope so... I meant to ask Gleb to take them while Alex was away, but
I forgot about them. :-P
Alex, are you back from vacation yet?
-Scott
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
