> > > > On 18.07.14 11:57, Bharat.Bhushan@xxxxxxxxxxxxx wrote: > > > > > >> -----Original Message----- > > >> From: Wood Scott-B07421 > > >> Sent: Friday, July 18, 2014 6:19 AM > > >> To: Alexander Graf > > >> Cc: Bhushan Bharat-R65777; kvm-ppc@xxxxxxxxxxxxxxx; > > >> kvm@xxxxxxxxxxxxxxx; 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@xxxxxxxxxxxxx wrote: > > >>>>>>>>>> -----Original Message----- > > >>>>>>>>>> From: Alexander Graf [mailto:agraf@xxxxxxx] > > >>>>>>>>>> Sent: Thursday, July 17, 2014 9:41 PM > > >>>>>>>>>> To: Bhushan Bharat-R65777; kvm-ppc@xxxxxxxxxxxxxxx > > >>>>>>>>>> Cc: kvm@xxxxxxxxxxxxxxx; 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@xxxxxxxxxxxxx> > > >>>>>>>>>>> --- > > >>>>>>>>>>> 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 ��.n��������+%������w��{.n�����o�^n�r������&��z�ޗ�zf���h���~����������_��+v���)ߣ�
