From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:38626 "EHLO mx0b-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726021AbgLDTI1 (ORCPT ); Fri, 4 Dec 2020 14:08:27 -0500 Date: Fri, 4 Dec 2020 20:05:02 +0100 From: Halil Pasic Subject: Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM pointer invalidated Message-ID: <20201204200502.1c34ae58.pasic@linux.ibm.com> In-Reply-To: References: <20201202234101.32169-1-akrowiak@linux.ibm.com> <20201203185514.54060568.pasic@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit List-ID: To: Tony Krowiak Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, borntraeger@de.ibm.com, cohuck@redhat.com, alex.williamson@redhat.com, kwankhede@nvidia.com, david@redhat.com, Janosch Frank On Fri, 4 Dec 2020 09:43:59 -0500 Tony Krowiak wrote: > >> +{ > >> + if (matrix_mdev->kvm) { > >> + (matrix_mdev->kvm); > >> + matrix_mdev->kvm->arch.crypto.pqap_hook = NULL; > > Is a plain assignment to arch.crypto.pqap_hook apropriate, or do we need > > to take more care? > > > > For instance kvm_arch_crypto_set_masks() takes kvm->lock before poking > > kvm->arch.crypto.crycb. > > I do not think so. The CRYCB is used by KVM to provide crypto resources > to the guest so it makes sense to protect it from changes to it while > passing > the AP devices through to the guest. The hook is used only when an AQIC > executed on the guest is intercepted by KVM. If the notifier > is being invoked to notify vfio_ap that KVM has been set to NULL, this means > the guest is gone in which case there will be no AP instructions to > intercept. If the update to pqap_hook isn't observed as atomic we still have a problem. With torn writes or reads we would try to use a corrupt function pointer. While the compiler probably ain't likely to generate silly code for the above assignment (multiple write instructions less then quadword wide), I know of nothing that would prohibit the compiler to do so. I'm not certain about the scope of the kvm->lock (if it's supposed to protect the whole sub-tree of objects). Maybe Janosch can help us out. @Janosch: what do you think? Regards, Halil