From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752847AbeBVI6X (ORCPT ); Thu, 22 Feb 2018 03:58:23 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:37276 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752553AbeBVI6W (ORCPT ); Thu, 22 Feb 2018 03:58:22 -0500 Date: Thu, 22 Feb 2018 08:58:10 +0000 Message-ID: <86tvu9jt99.wl-marc.zyngier@arm.com> From: Marc Zyngier To: Christoffer Dall Cc: Shanker Donthineni , linux-kernel , linux-arm-kernel , kvmarm , Vikram Sethi Subject: Re: [PATCH] KVM: arm/arm64: No need to zero CNTVOFF in kvm_timer_vcpu_put() for VHE In-Reply-To: <20180222084231.GA29376@cbox> References: <1519054687-14405-1-git-send-email-shankerd@codeaurora.org> <20180222084231.GA29376@cbox> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM/1.14.9 (=?ISO-8859-4?Q?Goj=F2?=) APEL/10.8 EasyPG/1.0.0 Emacs/25.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Organization: ARM Ltd MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=ISO-2022-JP Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 22 Feb 2018 08:42:31 +0000, Christoffer Dall wrote: > > Hi Shanker, > > On Mon, Feb 19, 2018 at 09:38:07AM -0600, Shanker Donthineni wrote: > > In AArch64/AArch32, the virtual counter uses a fixed virtual offset > > of zero in the following situations as per ARMv8 specifications: > > > > 1) HCR_EL2.E2H is 1, and CNTVCT_EL0/CNTVCT are read from EL2. > > 2) HCR_EL2.{E2H, TGE} is {1, 1}, and either: > > ― CNTVCT_EL0 is read from Non-secure EL0 or EL2. > > ― CNTVCT is read from Non-secure EL0. > > > > So, no need to zero CNTVOFF_EL2/CNTVOFF for VHE case. > > > > Signed-off-by: Shanker Donthineni > > --- > > virt/kvm/arm/arch_timer.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c > > index 70268c0..86eca324 100644 > > --- a/virt/kvm/arm/arch_timer.c > > +++ b/virt/kvm/arm/arch_timer.c > > @@ -541,9 +541,11 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu) > > * The kernel may decide to run userspace after calling vcpu_put, so > > * we reset cntvoff to 0 to ensure a consistent read between user > > * accesses to the virtual counter and kernel access to the physical > > - * counter. > > + * counter of non-VHE case. For VHE, the virtual counter uses a fixed > > + * virtual offset of zero, so no need to zero CNTVOFF_EL2 register. > > */ > > - set_cntvoff(0); > > + if (!has_vhe()) > > + set_cntvoff(0); > > } > > > > /* > > I'm okay with this change. I don't think there's a huge gain here > though. > > Marc, any thoughts or concerns? None. I don't think this brings any visible performance gain, but hey, why not. Acked-by: Marc Zyngier M. -- Jazz is not dead, it just smell funny.