RE: [PATCH] kvm: ppc: booke: Restore SPRG3 when entering guest

Bharat.Bhushan
2014-07-18T14:01:22Z

> >
> > On 18.07.14 11:57, Bharat.Bhushan@freescale.com wrote:
> > >
> > >> -----Original Message-----
> > >> From: Wood Scott-B07421
> > >> Sent: Friday, July 18, 2014 6:19 AM
> > >> To: Alexander Graf
> > >> Cc: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org;
> > >> kvm@vger.kernel.org; Yoder
> > >> Stuart-B08248
> > >> Subject: Re: [PATCH] kvm: ppc: booke: Restore SPRG3 when entering
> > >> guest
> > >>
> > >> On Fri, 2014-07-18 at 02:37 +0200, Alexander Graf wrote:
> > >>> On 18.07.14 02:36, Scott Wood wrote:
> > >>>> On Fri, 2014-07-18 at 02:33 +0200, Alexander Graf wrote:
> > >>>>> On 18.07.14 02:28, Scott Wood wrote:
> > >>>>>> On Thu, 2014-07-17 at 18:29 +0200, Alexander Graf wrote:
> > >>>>>>> On 17.07.14 18:27, Alexander Graf wrote:
> > >>>>>>>> On 17.07.14 18:24, Bharat.Bhushan@freescale.com wrote:
> > >>>>>>>>>> -----Original Message-----
> > >>>>>>>>>> From: Alexander Graf [mailto:agraf@suse.de]
> > >>>>>>>>>> Sent: Thursday, July 17, 2014 9:41 PM
> > >>>>>>>>>> To: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org
> > >>>>>>>>>> Cc: kvm@vger.kernel.org; Wood Scott-B07421; Yoder
> > >>>>>>>>>> Stuart-B08248
> > >>>>>>>>>> Subject: Re: [PATCH] kvm: ppc: booke: Restore SPRG3 when
> > >>>>>>>>>> entering guest
> > >>>>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>> On 16.07.14 08:02, Bharat Bhushan wrote:
> > >>>>>>>>>>> SPRG3 is guest accessible and SPRG3 can be clobbered by
> > >>>>>>>>>>> host or another guest, So this need to be restored when
> > >>>>>>>>>>> loading guest
> > >> state.
> > >>>>>> SPRG3 is not guest writeable. We should be doing this so that
> > >>>>>> guest reads of SPRG3 through the alternative read-only SPR
> > >>>>>> work, not because
> > >>>>>> "SPRG3 can be clobbered by host or another guest".
> > >>>>>>
> > >>>>>>>>>>> Signed-off-by: Bharat Bhushan
> > >>>>>>>>>>> <Bharat.Bhushan@freescale.com>
> > >>>>>>>>>>> ---
> > >>>>>>>>>>> arch/powerpc/kvm/booke_interrupts.S | 2 ++
> > >>>>>>>>>>> 1 file changed, 2 insertions(+)
> > >>>>>>>>>>>
> > >>>>>>>>>>> diff --git a/arch/powerpc/kvm/booke_interrupts.S
> > >>>>>>>>>>> b/arch/powerpc/kvm/booke_interrupts.S
> > >>>>>>>>>>> index 2c6deb5ef..0d3403f 100644
> > >>>>>>>>>>> --- a/arch/powerpc/kvm/booke_interrupts.S
> > >>>>>>>>>>> +++ b/arch/powerpc/kvm/booke_interrupts.S
> > >>>>>>>>>>> @@ -459,6 +459,8 @@ lightweight_exit:
> > >>>>>>>>>>> * written directly to the shared area, so we
> > >>>>>>>>>>> * need to reload them here with the guest's values.
> > >>>>>>>>>>> */
> > >>>>>>>>>>> + PPC_LD(r3, VCPU_SHARED_SPRG3, r5)
> > >>>>>>>>>>> + mtspr SPRN_SPRG3, r3
> > >>>>>>>>>> We also need to restore it when resuming the host, no?
> > >>>>>>>>> I do not think host expect some meaningful value when
> > >>>>>>>>> returning from guest, same true for SPRG4-7.
> > >>>>>>>>> So there seems no reason to save host values and restore them.
> > >>>>>> Linux no longer uses SPRG4-7 for itself. That is not true of
> > >>>>>> SPRG3, as Alex points out.
> > >>>>>>
> > >>>>>>>> Hmm - arch/powerpc/include/asm/reg.h says:
> > >>>>>>>>
> > >>>>>>>> * All 32-bit:
> > >>>>>>>> * - SPRG3 current thread_info pointer
> > >>>>>>>> * (virtual on BookE, physical on others)
> > >>>>>>>>
> > >>>>>>>> but I can indeed find no trace of usage anywhere. This at
> > >>>>>>>> least needs to go into the patch description.
> > >>>>>>> Bah - it obviously is used. It's SPRN_SPRG_THREAD. And it's so
> > >>>>>>> incredibly important that I have no idea how we could possibly
> > >>>>>>> run without switching the host value back in very early. And
> > >>>>>>> even then our interrupt handlers wouldn't work anymore.
> > >>>>>>>
> > >>>>>>> This is more complicated :).
> > >>>>>> To make this work we need to avoid SPRG3 as well, or at least
> > >>>>>> avoid using it for something needed prior to DO_KVM.
> > >>>>>>
> > >>>>>> We also need to update the documentation in reg.h to reflect
> > >>>>>> the fact that we don't use SPRG4-7 anymore on e500.
> > >>>>> I would personally prefer if we claim SPRG3R as unsupported on
> > >>>>> e500v2 until we find someone who actually uses it. There's a
> > >>>>> good chance we'd start jumping through a lot of hoops and reduce
> > >>>>> overall performance for no real-world gain today.
> > >>>> The same problem applies to e500mc.
> > > We have two SPRG3 registers
> > > 1) SPRN_SPRG3 -- All read/write access to this register in guest
> > will trap and emulate, So no need to save/restore.
> > > 2) SPRN_SPRG3R -- This is guest read only and We do not see any
> user
> > of this register, so can leave this for now
> > >
> > >>> There we have SPRN_GSPRG3, no?
> > >> Oh, right.
> > >>
> > >> Since it's only a problem for PR-mode, it can be fixed without
> > >> needing to avoid
> > >> SPRG3 entirely, since PR-mode doesn't use DO_KVM. We'd only need
> > >> to avoid using SPRG_THREAD in __KVM_HANDLER (i.e. revert commit
> > >> ffe129ecd79779221fdb03305049ec8b5a8beb0f).
> > > Did not get why using SPRG_THREAD here is a problem? Is not this
> > > will always access host SPRG3 and guest read write will always trap
> > > (maintained in vcpu->arch->shared->reg->sprg3)
> >
> > Guest reads via SPRG3R will access the real SPRG3 register. Guest
> > reads via
> > SPRG3 will trap :).
>
> Agree, Also we do not see Linux as guest is accessing SPRG3R. But what I did not
> get what the mentioned patch have to do with that?

Got it after discussion on IRC, If we need to care about SPRNG3R emulation then we should not use SPRG_THREAD .

Thanks
-Bharat
>
>
> >
> >
> > Alex