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 02:33:12 +0200 Message-ID: <53C86B48.6090402@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> 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: <1405643339.7714.46.camel@snotra.buserror.net> Sender: kvm-ppc-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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. Alex