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: Thu, 17 Jul 2014 18:29:43 +0200 Message-ID: <53C7F9F7.30800@suse.de> References: <1405490564-20119-1-git-send-email-Bharat.Bhushan@freescale.com> <53C7F593.2010804@suse.de> <53C7F981.9080405@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "kvm@vger.kernel.org" , Scott Wood , Stuart Yoder To: "Bharat.Bhushan@freescale.com" , "kvm-ppc@vger.kernel.org" Return-path: In-Reply-To: <53C7F981.9080405@suse.de> Sender: kvm-ppc-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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. >>>> >>>> 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. > > 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 :). Alex