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 62733C04A68 for ; Wed, 27 Jul 2022 19:16:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236424AbiG0TQH (ORCPT ); Wed, 27 Jul 2022 15:16:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33890 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236357AbiG0TPv (ORCPT ); Wed, 27 Jul 2022 15:15:51 -0400 Received: from mail-pl1-x635.google.com (mail-pl1-x635.google.com [IPv6:2607:f8b0:4864:20::635]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5B86D5C957 for ; Wed, 27 Jul 2022 12:04:40 -0700 (PDT) Received: by mail-pl1-x635.google.com with SMTP id x7so2927992pll.7 for ; Wed, 27 Jul 2022 12:04:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=nsmujacVytercybwEzTpO980C/0aSSMQ0B+3OnRKSQ0=; b=gdJhPtmUCwZmwlBzZ7mAbXflhOgEXZ6eNDtVjcumKvJjWlOPdN+SQptK0c8OogoJVL oAsqLe3SBrlIUStLB5mdx2RkKc17Bizexfsp0UNEtyZ6fmSDnxrIb3Jb2uzj1HBj9wCa TkcZPAseSbu9pxFaWGZXQRROJqcONQ/SDEMedqwYEkFNtfV7vEgiCuclrRj01iMXarIb OitGP1TduULw0yNiGl/JBj2w2N7Mj0QmLzonWrIZcytyuYEvIvLwlTGmQMYdEumyol0Q Ed5cwXUs/XQSFRyILPXOScInMTzhPALP71Zxnl3BTRw0VbGIULXDl4L861fHb4bRv70m L40g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=nsmujacVytercybwEzTpO980C/0aSSMQ0B+3OnRKSQ0=; b=giRFjEJilpOC1zvQDRg5RRElXAN1m0IQ9MhUGj5d8XusAjRGu4GndgItLtSsgG37dh xEM8gnx5fWt86fY1ThlwswbpcC1m8/Q1dMlrwhUvjpmKSco7Vrh2BMM98m4LljyqEjwf +4VEpTqs0i6q2bsWgKzMcaUVg0uysxpHJa6/d4OR2Q0BWxNzCk3pURHOBLHmEd02M4yx 8wYI3ZgnGMjsyHUKnHdee90OPwDwz8B4u4AqHqZXkvfbOSRlQTsxdCgOsTAcg52hZzBj SuIujfV0HtcqXgijJluVJbfH9j5aoToPaQKKh5RLZdlZLwWW+KzkA5EwMVSArBOR1X1N laCw== X-Gm-Message-State: AJIora9o9KPgFgOF0jB+C3MFSWLTPyWeEdobfhX7CkUmcR74uUHms2KJ HFVkXNXb+KN5Mhy1yUkghz4Hf9GjGi3LpQ== X-Google-Smtp-Source: AGRyM1v5jZAAYPCzTHY4R3UJnbQ1qTswvrQtXwoxO1hpUHUEC9cP5l/K0qZ9Co2B1258A01GzolK6A== X-Received: by 2002:a17:903:2343:b0:16c:1efe:1fe0 with SMTP id c3-20020a170903234300b0016c1efe1fe0mr23912094plh.106.1658948679661; Wed, 27 Jul 2022 12:04:39 -0700 (PDT) Received: from google.com (7.104.168.34.bc.googleusercontent.com. [34.168.104.7]) by smtp.gmail.com with ESMTPSA id c6-20020a170902d48600b0016dbe37cebdsm467135plg.246.2022.07.27.12.04.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 Jul 2022 12:04:39 -0700 (PDT) Date: Wed, 27 Jul 2022 19:04:35 +0000 From: Sean Christopherson To: Yan Zhao Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Yosry Ahmed , Mingwei Zhang , Ben Gardon Subject: Re: [PATCH v2 4/6] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages Message-ID: References: <20220723012325.1715714-1-seanjc@google.com> <20220723012325.1715714-5-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 27, 2022, Yan Zhao wrote: > On Sat, Jul 23, 2022 at 01:23:23AM +0000, Sean Christopherson wrote: > > > > > @@ -386,16 +385,18 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn, > > static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp, > > bool shared) > > { > > + atomic64_dec(&kvm->arch.tdp_mmu_pages); > > + > > + if (!sp->nx_huge_page_disallowed) > > + return; > > + > Does this read of sp->nx_huge_page_disallowed also need to be protected by > tdp_mmu_pages_lock in shared path? No, because only one CPU can call tdp_mmu_unlink_sp() for a shadow page. E.g. in a shared walk, the SPTE is zapped atomically and only the CPU that "wins" gets to unlink the s[. The extra lock is needed to prevent list corruption, but the sp itself is thread safe. FWIW, even if that guarantee didn't hold, checking the flag outside of tdp_mmu_pages_lock is safe because false positives are ok. untrack_possible_nx_huge_page() checks that the shadow page is actually on the list, i.e. it's a nop if a different task unlinks the page first. False negatives need to be avoided, but nx_huge_page_disallowed is cleared only when untrack_possible_nx_huge_page() is guaranteed to be called, i.e. true false negatives can't occur. Hmm, but I think there's a missing smp_rmb(), which is needed to ensure nx_huge_page_disallowed is read after observing the shadow-present SPTE (that's being unlinked). I'll add that in the next version.