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 50FEFC433EF for ; Mon, 13 Dec 2021 12:53:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233353AbhLMMxL (ORCPT ); Mon, 13 Dec 2021 07:53:11 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45012 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231509AbhLMMxK (ORCPT ); Mon, 13 Dec 2021 07:53:10 -0500 Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F25C5C061574 for ; Mon, 13 Dec 2021 04:53:09 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id 47951CE0FF7 for ; Mon, 13 Dec 2021 12:53:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9B0DBC34603; Mon, 13 Dec 2021 12:53:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639399986; bh=BxpFYEalQW7I1obrELsCap0ySUis6eH6NC+wPVvjF5c=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ZPDlGBfx6aounopVqMoIeB7/Km5GVGb4un4MmsPLF0z0Jow4ee3Fin+xyvotkZjna +HQXYWO7CVHglWL2uH85pacYkVxzgCsNEujYRYb+xUotOZNeh96hUsw/dX3Q1hzUWi pC6IuPHdDHrPdzrR7328EnFsoMBrOXRp+hh58MtW3YqFfbpbcPd81QytVcW9CWaBES eKK4CoflTQnLfuj64O5Q3A/GKtzVNbPDmTNkc+jtIS88Uj/d2vVJlCiLaKWHFFcyNg MqX5q4kkMxxyLMcQ1jVzCd6De2QF2xGEZNuyuA3F4XDYPiIKO2IMar+abiCrki7yfK 3/rSlp8TN4z6g== Date: Mon, 13 Dec 2021 12:53:00 +0000 From: Will Deacon To: Quentin Perret Cc: Marc Zyngier , James Morse , Alexandru Elisei , Suzuki K Poulose , Catalin Marinas , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages Message-ID: <20211213125300.GA10915@willie-the-truck> References: <20211201170411.1561936-1-qperret@google.com> <20211201170411.1561936-4-qperret@google.com> <20211209102924.GC1833@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 10, 2021 at 02:34:16PM +0000, Quentin Perret wrote: > On Thursday 09 Dec 2021 at 10:29:24 (+0000), Will Deacon wrote: > > On Wed, Dec 01, 2021 at 05:03:57PM +0000, Quentin Perret wrote: > > > To prepare the ground for allowing hyp stage-1 mappings to be removed at > > > run-time, update the KVM page-table code to maintain a correct refcount > > > using the ->{get,put}_page() function callbacks. > > > > > > Signed-off-by: Quentin Perret > > > --- > > > arch/arm64/kvm/hyp/pgtable.c | 17 ++++++++++++++--- > > > 1 file changed, 14 insertions(+), 3 deletions(-) > > > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > > index f8ceebe4982e..768a58835153 100644 > > > --- a/arch/arm64/kvm/hyp/pgtable.c > > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > > @@ -408,8 +408,10 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, > > > return false; > > > > > > new = kvm_init_valid_leaf_pte(phys, data->attr, level); > > > - if (hyp_pte_needs_update(old, new)) > > > + if (hyp_pte_needs_update(old, new)) { > > > smp_store_release(ptep, new); > > > + data->mm_ops->get_page(ptep); > > > > In the case where we're just updating software bits for a valid pte, doesn't > > this result in us taking a spurious reference to the page? > > Ahem, yes, that is the case. I ended up with the below diff to fix it, > which I intend to fold in the next version: > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 6ad4cb2d6947..e2047d3f05a2 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -383,21 +383,6 @@ enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte) > return prot; > } > > -static bool hyp_pte_needs_update(kvm_pte_t old, kvm_pte_t new) > -{ > - /* > - * Tolerate KVM recreating the exact same mapping, or changing software > - * bits if the existing mapping was valid. > - */ > - if (old == new) > - return false; > - > - if (!kvm_pte_valid(old)) > - return true; > - > - return !WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW); > -} > - > static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, > kvm_pte_t *ptep, struct hyp_map_data *data) > { > @@ -407,13 +392,16 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, > if (!kvm_block_mapping_supported(addr, end, phys, level)) > return false; > > + data->phys += granule; > new = kvm_init_valid_leaf_pte(phys, data->attr, level); > - if (hyp_pte_needs_update(old, new)) { > - smp_store_release(ptep, new); > + if (old == new) > + return true; > + else if (!kvm_pte_valid(old)) (nit: clearer to drop the 'else' part here) > data->mm_ops->get_page(ptep); Ok, so this works because new is always valid. LGTM. Will