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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 82374ECAAD3 for ; Fri, 9 Sep 2022 09:56:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229761AbiIIJ4r (ORCPT ); Fri, 9 Sep 2022 05:56:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58940 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229821AbiIIJ4f (ORCPT ); Fri, 9 Sep 2022 05:56:35 -0400 Received: from out1.migadu.com (out1.migadu.com [IPv6:2001:41d0:2:863f::]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BE5C112D19E; Fri, 9 Sep 2022 02:56:10 -0700 (PDT) Date: Fri, 9 Sep 2022 10:55:59 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1662717368; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lDpC67NopcBY3WkfHCdst9Y40eb0+fWa3aZaj8C1P74=; b=LVUcgLVEKlJeyJh274QBbSm2SunUsd/+Fnhi8Oa83s4LnsBzI0E2D1UEwYFcXtQEulUU+W LVnoIby+wN2xFlyIPs/26HCtShwG9QsGAppu7ff2QUan911DOy14slnZlOYJWqyTLNYjZD NI3YBiakBpwSB/KZjauRUKa3/8sbkjE= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: David Matlack Cc: Marc Zyngier , James Morse , Alexandru Elisei , Suzuki K Poulose , Catalin Marinas , Will Deacon , Quentin Perret , Ricardo Koller , Reiji Watanabe , Ben Gardon , Paolo Bonzini , Gavin Shan , Peter Xu , Sean Christopherson , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 08/14] KVM: arm64: Protect page table traversal with RCU Message-ID: References: <20220830194132.962932-1-oliver.upton@linux.dev> <20220830194132.962932-9-oliver.upton@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Sep 07, 2022 at 02:47:08PM -0700, David Matlack wrote: > On Tue, Aug 30, 2022 at 07:41:26PM +0000, Oliver Upton wrote: > > The use of RCU is necessary to change the paging structures in parallel. > > Acquire and release an RCU read lock when traversing the page tables. > > > > Signed-off-by: Oliver Upton > > --- > > arch/arm64/include/asm/kvm_pgtable.h | 19 ++++++++++++++++++- > > arch/arm64/kvm/hyp/pgtable.c | 7 ++++++- > > 2 files changed, 24 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > > index 78fbb7be1af6..7d2de0a98ccb 100644 > > --- a/arch/arm64/include/asm/kvm_pgtable.h > > +++ b/arch/arm64/include/asm/kvm_pgtable.h > > @@ -578,9 +578,26 @@ enum kvm_pgtable_prot kvm_pgtable_stage2_pte_prot(kvm_pte_t pte); > > */ > > enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte); > > > > +#if defined(__KVM_NVHE_HYPERVISOR___) > > + > > Future readers will wonder why NVHE stubs out RCU support and how that > is even correct. Some comments here would be useful explain it. Good point. > > +static inline void kvm_pgtable_walk_begin(void) {} > > +static inline void kvm_pgtable_walk_end(void) {} > > + > > +#define kvm_dereference_ptep rcu_dereference_raw > > How does NVHE have access rcu_dereference_raw()? rcu_dereference_raw() is inlined and simply recasts the pointer into the kernel address space. Perhaps it is less confusing to template this on kvm_pte_read() to avoid polluting nVHE with an otherwise benign reference to RCU. > > + > > +#else /* !defined(__KVM_NVHE_HYPERVISOR__) */ > > + > > +#define kvm_pgtable_walk_begin rcu_read_lock > > +#define kvm_pgtable_walk_end rcu_read_unlock > > +#define kvm_dereference_ptep rcu_dereference > > + > > +#endif /* defined(__KVM_NVHE_HYPERVISOR__) */ > > + > > static inline kvm_pte_t kvm_pte_read(kvm_pte_t *ptep) > > { > > - return READ_ONCE(*ptep); > > + kvm_pte_t __rcu *p = (kvm_pte_t __rcu *)ptep; > > + > > + return READ_ONCE(*kvm_dereference_ptep(p)); > > What about all the other places where page table memory is accessed? > > If RCU is going to be used to protect page table memory, then all > accesses have to go under an RCU critical section. This means that page > table memory should only be accessed through __rcu annotated pointers > and dereferenced with rcu_dereference(). Let me play around with this a bit, as the annoying part is trying to sprinkle in RCU annotations w/o messing with nVHE. -- Thanks, Oliver