From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Graf Subject: Re: [PATCH] kvm: ppc: booke: Restore SPRG3 when entering guest Date: Fri, 18 Jul 2014 02:37:13 +0200 Message-ID: <53C86C39.7020609@suse.de> References: <1405490564-20119-1-git-send-email-Bharat.Bhushan@freescale.com> <53C7F593.2010804@suse.de> <53C7F981.9080405@suse.de> <53C7F9F7.30800@suse.de> <1405643339.7714.46.camel@snotra.buserror.net> <53C86B48.6090402@suse.de> <1405643769.7714.47.camel@snotra.buserror.net> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: "Bharat.Bhushan@freescale.com" , "kvm-ppc@vger.kernel.org" , "kvm@vger.kernel.org" , Stuart Yoder To: Scott Wood Return-path: In-Reply-To: <1405643769.7714.47.camel@snotra.buserror.net> Sender: kvm-ppc-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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 >>>>>>>> --- >>>>>>>> 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. There we have SPRN_GSPRG3, no? Alex