From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BF604C433E1 for ; Mon, 22 Jun 2020 09:51:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A0536206F1 for ; Mon, 22 Jun 2020 09:51:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1592819513; bh=rVF+9gcAfBgoO5QMjVC82HmvyQAsTljKLIObnB7+V7k=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=urZkm88pC2VxLSL8QVislKnB8IrYzN0Jw/KrVkGeNk5iX94eEZXLrUg6A7UpEZUR4 eLtvKV6rHHWjIbIuUWiVxKeenqoX6fhMEGtzoHIdniaOFsGqT2M5sOnmlPKlAgWKY8 Xl7iuOX5BTEQ450kH7ciiqKqw7xCdEVvHTW0BVtQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727091AbgFVJvw (ORCPT ); Mon, 22 Jun 2020 05:51:52 -0400 Received: from mail.kernel.org ([198.145.29.99]:34196 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726901AbgFVJvu (ORCPT ); Mon, 22 Jun 2020 05:51:50 -0400 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 4A3F3206F1; Mon, 22 Jun 2020 09:51:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1592819509; bh=rVF+9gcAfBgoO5QMjVC82HmvyQAsTljKLIObnB7+V7k=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=xlvC8uLW6kj3gouL5RXCKfImri7AfcHGBH7ugeHRWCyySAi2UdREYdkVLjAxs0ypt HS6RL0eqK9MgSN9uxYIrhhZnemfCoux7AjmUJfIc5VcObBCfJH7dVboV4RgUglNIw3 /0g/M5fcpGdzIwV8PK2mFc/VIo2CF8OEjl/gwyXU= Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1jnJ7D-005H5Q-QD; Mon, 22 Jun 2020 10:51:47 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Mon, 22 Jun 2020 10:51:47 +0100 From: Marc Zyngier To: Andrew Jones Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, pbonzini@redhat.com, steven.price@arm.com Subject: Re: [PATCH 2/4] arm64/x86: KVM: Introduce steal time cap In-Reply-To: <20200622084110.uosiqx3oy22lremu@kamzik.brq.redhat.com> References: <20200619184629.58653-1-drjones@redhat.com> <20200619184629.58653-3-drjones@redhat.com> <5b1e895dc0c80bef3c0653894e2358cf@kernel.org> <20200622084110.uosiqx3oy22lremu@kamzik.brq.redhat.com> User-Agent: Roundcube Webmail/1.4.5 Message-ID: <5a52210e5f123d52459f15c594e77bad@kernel.org> X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: drjones@redhat.com, kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, pbonzini@redhat.com, steven.price@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 2020-06-22 09:41, Andrew Jones wrote: > On Mon, Jun 22, 2020 at 09:20:02AM +0100, Marc Zyngier wrote: >> Hi Andrew, >> >> On 2020-06-19 19:46, Andrew Jones wrote: >> > arm64 requires a vcpu fd (KVM_HAS_DEVICE_ATTR vcpu ioctl) to probe >> > support for steal time. However this is unnecessary and complicates >> > userspace (userspace may prefer delaying vcpu creation until after >> > feature probing). Since probing steal time only requires a KVM fd, >> > we introduce a cap that can be checked. >> >> So this is purely an API convenience, right? You want a way to >> identify the presence of steal time accounting without having to >> create a vcpu? It would have been nice to have this requirement >> before we merged this code :-(. > > Yes. I wish I had considered it more closely when I was reviewing the > patches. And, I believe we have yet another user interface issue that > I'm looking at now. Without the VCPU feature bit I'm not sure how easy > it will be for a migration to fail when attempting to migrate from a > host > with steal-time enabled to one that does not support steal-time. So > it's > starting to look like steal-time should have followed the pmu pattern > completely, not just the vcpu device ioctl part. Should we consider disabling steal time altogether until this is worked out? >> >> > Additionally, when probing >> > steal time we should check delayacct_on, because even though >> > CONFIG_KVM selects TASK_DELAY_ACCT, it's possible for the host >> > kernel to have delay accounting disabled with the 'nodelayacct' >> > command line option. x86 already determines support for steal time >> > by checking delayacct_on and can already probe steal time support >> > with a kvm fd (KVM_GET_SUPPORTED_CPUID), but we add the cap there >> > too for consistency. >> > >> > Signed-off-by: Andrew Jones >> > --- >> > Documentation/virt/kvm/api.rst | 11 +++++++++++ >> > arch/arm64/kvm/arm.c | 3 +++ >> > arch/x86/kvm/x86.c | 3 +++ >> > include/uapi/linux/kvm.h | 1 + >> > 4 files changed, 18 insertions(+) >> > >> > diff --git a/Documentation/virt/kvm/api.rst >> > b/Documentation/virt/kvm/api.rst >> > index 9a12ea498dbb..05b1fdb88383 100644 >> > --- a/Documentation/virt/kvm/api.rst >> > +++ b/Documentation/virt/kvm/api.rst >> > @@ -6151,3 +6151,14 @@ KVM can therefore start protected VMs. >> > This capability governs the KVM_S390_PV_COMMAND ioctl and the >> > KVM_MP_STATE_LOAD MP_STATE. KVM_SET_MP_STATE can fail for protected >> > guests when the state change is invalid. >> > + >> > +8.24 KVM_CAP_STEAL_TIME >> > +----------------------- >> > + >> > +:Architectures: arm64, x86 >> > + >> > +This capability indicates that KVM supports steal time accounting. >> > +When steal time accounting is supported it may be enabled with >> > +architecture-specific interfaces. For x86 see >> > +Documentation/virt/kvm/msr.rst "MSR_KVM_STEAL_TIME". For arm64 see >> > +Documentation/virt/kvm/devices/vcpu.rst "KVM_ARM_VCPU_PVTIME_CTRL" >> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c >> > index 90cb90561446..f6dca6d09952 100644 >> > --- a/arch/arm64/kvm/arm.c >> > +++ b/arch/arm64/kvm/arm.c >> > @@ -222,6 +222,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, >> > long ext) >> > */ >> > r = 1; >> > break; >> > + case KVM_CAP_STEAL_TIME: >> > + r = sched_info_on(); >> > + break; >> > default: >> > r = kvm_arch_vm_ioctl_check_extension(kvm, ext); >> > break; >> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> > index 00c88c2f34e4..ced6335e403e 100644 >> > --- a/arch/x86/kvm/x86.c >> > +++ b/arch/x86/kvm/x86.c >> > @@ -3533,6 +3533,9 @@ int kvm_vm_ioctl_check_extension(struct kvm >> > *kvm, long ext) >> > case KVM_CAP_HYPERV_ENLIGHTENED_VMCS: >> > r = kvm_x86_ops.nested_ops->enable_evmcs != NULL; >> > break; >> > + case KVM_CAP_STEAL_TIME: >> > + r = sched_info_on(); >> > + break; >> > default: >> > break; >> > } >> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >> > index 4fdf30316582..121fb29ac004 100644 >> > --- a/include/uapi/linux/kvm.h >> > +++ b/include/uapi/linux/kvm.h >> > @@ -1031,6 +1031,7 @@ struct kvm_ppc_resize_hpt { >> > #define KVM_CAP_PPC_SECURE_GUEST 181 >> > #define KVM_CAP_HALT_POLL 182 >> > #define KVM_CAP_ASYNC_PF_INT 183 >> > +#define KVM_CAP_STEAL_TIME 184 >> > >> > #ifdef KVM_CAP_IRQ_ROUTING >> >> Shouldn't you also add the same check of sched_info_on() to >> the various pvtime attribute handling functions? It feels odd >> that the capability can say "no", and yet we'd accept userspace >> messing with the steal time parameters... > > I considered that, but the 'has attr' interface is really only asking > if the interface exists, not if it should be used. I'm not sure what > we should do about it other than document that the cap needs to say > it's usable, rather than just the attr presence. But, since we've had > the attr merged quite a while without the cap, then maybe we can't > rely on a doc change alone? Accepting the pvtime attributes (setting up the per-vcpu area) has two effects: we promise both the guest and userspace that we will provide the guest with steal time. By not checking sched_info_on(), we lie to both, with potential consequences. It really feels like a bug. Thanks, M. -- Jazz is not dead. It just smells funny...