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. -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
