> -----Original Message----- > From: Wood Scott-B07421 > Sent: Friday, July 18, 2014 6:19 AM > To: Alexander Graf > Cc: Bhushan Bharat-R65777; kvm-...@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.bhus...@freescale.com wrote: > > >>>>>>> -----Original Message----- > > >>>>>>> From: Alexander Graf [mailto:ag...@suse.de] > > >>>>>>> Sent: Thursday, July 17, 2014 9:41 PM > > >>>>>>> To: Bhushan Bharat-R65777; kvm-...@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.bhus...@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) Yes we are not consistent with "KVM, the host SPRG1 is used to point to the current VCPU data structure" , which need to be corrected Thanks -Bharat > > And if we decide it's not worthwhile and don't revert that commit, we should > at > least remove the comment that "Under KVM, the host SPRG1 is used to point to > the > current VCPU data structure"... > > -Scott >
