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 948FEC77B7F for ; Mon, 8 May 2023 14:05:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234380AbjEHOFw (ORCPT ); Mon, 8 May 2023 10:05:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44566 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234411AbjEHOFs (ORCPT ); Mon, 8 May 2023 10:05:48 -0400 Received: from mail-yb1-xb49.google.com (mail-yb1-xb49.google.com [IPv6:2607:f8b0:4864:20::b49]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 90F1238F28 for ; Mon, 8 May 2023 07:05:40 -0700 (PDT) Received: by mail-yb1-xb49.google.com with SMTP id 3f1490d57ef6-b9a6eeea78cso34288138276.0 for ; Mon, 08 May 2023 07:05:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1683554739; x=1686146739; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=jcl8cYiE8GBONZ/kHylHxTzdCz6iJZwPGQYSWy24JYA=; b=o8mfj/R750kMfA2bvKpyJZuf/ZhVeSnXBF4/wapKjmLIWy2rqeeSrAOt5X+YjYxuNh OyItG8a6SQBaYR3ol78KhlI6eh/Q0yN90HXJhxbVGAtpEUWsNrYhxsckDEtq7PX0t1EO f0JB4uXPde4Ox7tQXsrWT/ya3dTEG/JI0bIE5xqIVrID3KZMsDfqqnqBT9Za2r4APAZX B2RpVJzivT+iO2b+LbU+0Zontf8zI/Ed9ngpy3ey89L83c7xn7vHODjkZU8knTdQD3t/ n78u+Am85L+2PUzMWncpwoOAYV0GlSklrJfhHVJlIR1I82Kkd1Qrpj4q6M6s9sph6PlJ aA7A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1683554739; x=1686146739; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=jcl8cYiE8GBONZ/kHylHxTzdCz6iJZwPGQYSWy24JYA=; b=jAxM/jjfaOcfItfG9It7XA8rFj1cdxAWxDnU1M2/54xhxQmgUsIJgXLZPRTcfEe3vx Yi/0hspXAVjdr0n/2nUA8QxLmbBIKtpGad+0peu8+80n0R/jDKNOsDRL4GAJFdjzXoW+ bdATAd1DGJOC9Qiz2PfQO41e/7lPfBFILgQ3BCLBPfe/3yK7sKHW4CXdP7LvCuyvXl1v +ev842ooVSqrg2rwZ/dWjE3lZDt6xDTF5XzCo/+daScdwWBI9PpEC2xnOK6LzCoc0ETQ u4Zp9aVDg3ymMAxBSOp8Aa5by0eCETD9Mnl18XZ4U0hDJERZ3N48jNmmcd0+y1x6bsLk aT3g== X-Gm-Message-State: AC+VfDyNosus1bCW/tcumeGmC1aieVvpFpNwNAToVamZyQsh6BX+wufw c+LxreFkHj/lKj1vZVXSEDuUVM4dRls= X-Google-Smtp-Source: ACHHUZ7FCpMclh8b9aJYyQEcgnuYE0OwetS8XqokoS1TJPz66KwW2xxY/XhdC86wb/WWcOzl5JiJiPlwO6k= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:690c:10c:b0:55c:a5db:869 with SMTP id bd12-20020a05690c010c00b0055ca5db0869mr10528119ywb.4.1683554739792; Mon, 08 May 2023 07:05:39 -0700 (PDT) Date: Mon, 8 May 2023 07:05:32 -0700 In-Reply-To: Mime-Version: 1.0 References: <20230311002258.852397-1-seanjc@google.com> <20230311002258.852397-6-seanjc@google.com> Message-ID: Subject: Re: [PATCH v2 05/27] drm/i915/gvt: Verify VFIO-pinned page is THP when shadowing 2M gtt entry From: Sean Christopherson To: Yan Zhao Cc: kvm@vger.kernel.org, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, Zhenyu Wang , Ben Gardon , Paolo Bonzini , intel-gvt-dev@lists.freedesktop.org, Zhi Wang Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Sat, May 06, 2023, Yan Zhao wrote: > On Sat, May 06, 2023 at 02:35:41PM +0800, Yan Zhao wrote: > > > > Maybe the checking of PageTransHuge(cur_page) and bailing out is not necessary. > > > > If a page is not transparent huge, but there are 512 contigous 4K > > > > pages, I think it's still good to map them in IOMMU in 2M. > > > > See vfio_pin_map_dma() who does similar things. > > > > > > I agree that bailing isn't strictly necessary, and processing "blindly" should > > > Just Work for HugeTLB and other hugepage types. I was going to argue that it > > > would be safer to add this and then drop it at the end, but I think that's a > > > specious argument. If not checking the page type is unsafe, then the existing > > > code is buggy, and this changelog literally states that the check for contiguous > > > pages guards against any such problems. > > > > > > I do think there's a (very, very theoretical) issue though. For "CONFIG_SPARSEMEM=y > > > && CONFIG_SPARSEMEM_VMEMMAP=n", struct pages aren't virtually contiguous with respect > > > to their pfns, i.e. it's possible (again, very theoretically) that two struct pages > > > could be virtually contiguous but physically discontiguous. I suspect I'm being > > > ridiculously paranoid, but for the efficient cases where pages are guaranteed to > > > be contiguous, the extra page_to_pfn() checks should be optimized away by the > > > compiler, i.e. there's no meaningful downside to the paranoia. > > To make sure I understand it correctly: > > There are 3 conditions: > > (1) Two struct pages aren't virtually contiguous, but there PFNs are contiguous. > > (2) Two struct pages are virtually contiguous but their PFNs aren't contiguous. > > (Looks this will not happen?) > > (3) Two struct pages are virtually contiguous, and their PFNs are contiguous, too. > > But they have different backends, e.g. > > PFN 1 and PFN 2 are contiguous, while PFN 1 belongs to RAM, and PFN 2 > > belongs to DEVMEM. > > > > I think you mean condition (3) is problematic, am I right? > Oh, I got it now. > You are saying about condition (2), with "CONFIG_SPARSEMEM=y && > CONFIG_SPARSEMEM_VMEMMAP=n". > Two struct pages are contiguous if one is at one section's tail and another at > another section's head, but the two sections aren't for contiguous PFNs. Yep, exactly.