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 155D5C433F5 for ; Mon, 22 Nov 2021 21:48:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235247AbhKVVvM (ORCPT ); Mon, 22 Nov 2021 16:51:12 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234057AbhKVVvM (ORCPT ); Mon, 22 Nov 2021 16:51:12 -0500 Received: from mail-il1-x136.google.com (mail-il1-x136.google.com [IPv6:2607:f8b0:4864:20::136]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3F5F7C061574 for ; Mon, 22 Nov 2021 13:48:05 -0800 (PST) Received: by mail-il1-x136.google.com with SMTP id i9so14109416ilu.1 for ; Mon, 22 Nov 2021 13:48:05 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=cUC78J9wnQRA3xZgTEIrovLm14ZaQAPvQlLV0U3n2pw=; b=IT5jT8xzVfVGW6Mg1OOKSuNBTjB5AxkIvoS5Gl0u42kXXs4oP+xw21Z8ZHe4TeNaf/ YIxy+giqHsn9DArXeYwptZPg62aL7WHvrjVDjRMCPWglGdcuaMjDGROwVJuY42TCV0y4 3JfzZGRGEQDZOsfxIhWF/x2h9vBh500cRRoW83ff6Pq2WaVhXFnkOZdR5OtBOUY7OFeN XzpiM/yKFK7A6mqYKuN6ak/jSlf3n5joGkcfFiETt0E6vQoozvQXXIFautgmRH/dFxUQ omXJIJ5zCYGHwqDkkmoyTXtRlQEiXlDguA7I9iHoKuUOPjElC+pcRbSOtx/OKQefGN0X LiWA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=cUC78J9wnQRA3xZgTEIrovLm14ZaQAPvQlLV0U3n2pw=; b=HK9z/sbgmrKIV1YCdgaABYK7k3hjI3MiNs/Knd/9SR6A/e9XzHlphx+1+m+1csROch LY07L1wwfYLpvcb6v/UWwvbDroAWu3pWwqX9L1cgNj3js1D0tp2nxntTuEHubkiUW4fr jZ/D5GrCsWJPwXsTdvdGwd6/SAxQnje69psH4c332q4lqv64PMCoIOvJ3TSucbFJxMfo kmsaO0bkB5m/cAHNZyZRJ/WI77gZ2kKU9Z5r+4H132dj90zBp2qsW6d7BB7m4f8yphka saN2AyA2v8yYqBhR8WxR9sn/SX3m/zP2Fc8RIHm0sKvLQ3PFGihKRTQgZJzXeDNse3LT XO4g== X-Gm-Message-State: AOAM530e+bzmCs8PzFZxLhQWjRa5aTtx7KLPi/MhpSLA6il088nmUoYS WS4Xflktjk3jwCMB2ak2Y3JfiuQy9cmcQ41Xca9R5w== X-Google-Smtp-Source: ABdhPJzzT5CLp/xkYMUjDKH3WpZQW3gASaEDHJx3UvFMjqMssQrQnFx5SlykLjKhKNwT9fHnMlGwWUATHUYWkb1YQBY= X-Received: by 2002:a92:dccc:: with SMTP id b12mr130966ilr.129.1637617684507; Mon, 22 Nov 2021 13:48:04 -0800 (PST) MIME-Version: 1.0 References: <20211120045046.3940942-1-seanjc@google.com> <20211120045046.3940942-14-seanjc@google.com> In-Reply-To: <20211120045046.3940942-14-seanjc@google.com> From: Ben Gardon Date: Mon, 22 Nov 2021 13:47:53 -0800 Message-ID: Subject: Re: [PATCH 13/28] KVM: x86/mmu: Drop RCU after processing each root in MMU notifier hooks To: Sean Christopherson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Hou Wenlong Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson wrote: > > Drop RCU protection after processing each root when handling MMU notifier > hooks that aren't the "unmap" path, i.e. aren't zapping. Temporarily > drop RCU to let RCU do its thing between roots, and to make it clear that > there's no special behavior that relies on holding RCU across all roots. > > Currently, the RCU protection is completely superficial, it's necessary > only to make rcu_dereference() of SPTE pointers happy. A future patch > will rely on holding RCU as a proxy for vCPUs in the guest, e.g. to > ensure shadow pages aren't freed before all vCPUs do a TLB flush (or > rather, acknowledge the need for a flush), but in that case RCU needs to > be held until the flush is complete if and only if the flush is needed > because a shadow page may have been removed. And except for the "unmap" > path, MMU notifier events cannot remove SPs (don't toggle PRESENT bit, > and can't change the PFN for a SP). > > Signed-off-by: Sean Christopherson Reviewed-by: Ben Gardon > --- > arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index 06b500fab248..3ff7b4cd7d0e 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -1079,18 +1079,19 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm, > > lockdep_assert_held_write(&kvm->mmu_lock); > > - rcu_read_lock(); > - > /* > * Don't support rescheduling, none of the MMU notifiers that funnel > * into this helper allow blocking; it'd be dead, wasteful code. > */ > for_each_tdp_mmu_root(kvm, root, range->slot->as_id) { > + rcu_read_lock(); > + > tdp_root_for_each_leaf_pte(iter, root, range->start, range->end) > ret |= handler(kvm, &iter, range); > + > + rcu_read_unlock(); > } > > - rcu_read_unlock(); > > return ret; > } > -- > 2.34.0.rc2.393.gf8c9666880-goog >