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 gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0E4D6C00140 for ; Mon, 15 Aug 2022 10:18:56 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id B9D73A98F6; Mon, 15 Aug 2022 10:18:37 +0000 (UTC) Received: from madras.collabora.co.uk (madras.collabora.co.uk [46.235.227.172]) by gabe.freedesktop.org (Postfix) with ESMTPS id EEEFF8E7E3 for ; Mon, 15 Aug 2022 10:18:14 +0000 (UTC) Received: from [IPV6:2a00:5f00:102:0:10b3:10ff:fe5d:4ec1] (unknown [IPv6:2a00:5f00:102:0:10b3:10ff:fe5d:4ec1]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)) (No client certificate requested) (Authenticated sender: dmitry.osipenko) by madras.collabora.co.uk (Postfix) with ESMTPSA id 8DF3B66016A1; Mon, 15 Aug 2022 11:18:12 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1660558693; bh=bt6zHiqFOssYaXdPmgRbYknys43UYciOnxf9yqFEAiI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=hgAo6geyIhflayb4fSNhoE0iCQz9PLhgoSSF3k70UyLRnrFgpztGxMJg5NTLlbrHn Boy7/1AsUR7rYYjFtOzLYzOV0aaHSviLrmzom9aJdFWAtTnLlEH2YpbL4cKtSXt5dl e5IgECy+yDiKVh5hx4Rct6WzBM78FtN1fTxgnm48704p8dhndVSPuzKi89qJ5/PyjR /33/QfZmcYn0KcI7HU9fVdZKdQChWY++Jn2YlT7UkydJatUawuEud1tdsAPQcEHwB8 KaR4zimR74GNPao0P+Y/5iMGrQO7UCmhRPCO89DBM92q7hvRscTboHWQHSKTH6wkxm r9kAFPouLjDVA== Message-ID: <14be3b22-1d60-732b-c695-ddacc6b21055@collabora.com> Date: Mon, 15 Aug 2022 13:18:10 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages Content-Language: en-US To: =?UTF-8?Q?Christian_K=c3=b6nig?= , David Airlie , Huang Rui , Daniel Vetter , Trigger Huang , Gert Wollny , Antonio Caggiano References: <20220815095423.11131-1-dmitry.osipenko@collabora.com> <8230a356-be38-f228-4a8e-95124e8e8db6@amd.com> <134bce02-58d6-8553-bb73-42dfda18a595@collabora.com> <8caf3008-dcf3-985a-631e-e019b277c6f0@amd.com> <4fcc4739-2da9-1b89-209c-876129604d7d@amd.com> From: Dmitry Osipenko In-Reply-To: <4fcc4739-2da9-1b89-209c-876129604d7d@amd.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, virtualization@lists.linux-foundation.org, Dmitry Osipenko , kernel@collabora.com Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On 8/15/22 13:14, Christian König wrote: > Am 15.08.22 um 12:11 schrieb Christian König: >> Am 15.08.22 um 12:09 schrieb Dmitry Osipenko: >>> On 8/15/22 13:05, Christian König wrote: >>>> Am 15.08.22 um 11:54 schrieb Dmitry Osipenko: >>>>> Higher order pages allocated using alloc_pages() aren't refcounted and >>>>> they >>>>> need to be refcounted, otherwise it's impossible to map them by >>>>> KVM. This >>>>> patch sets the refcount of the tail pages and fixes the KVM memory >>>>> mapping >>>>> faults. >>>>> >>>>> Without this change guest virgl driver can't map host buffers into >>>>> guest >>>>> and can't provide OpenGL 4.5 profile support to the guest. The host >>>>> mappings are also needed for enabling the Venus driver using host GPU >>>>> drivers that are utilizing TTM. >>>>> >>>>> Based on a patch proposed by Trigger Huang. >>>> Well I can't count how often I have repeated this: This is an >>>> absolutely >>>> clear NAK! >>>> >>>> TTM pages are not reference counted in the first place and because of >>>> this giving them to virgl is illegal. >>> A? The first page is refcounted when allocated, the tail pages are not. >> >> No they aren't. The first page is just by coincident initialized with >> a refcount of 1. This refcount is completely ignored and not used at all. >> >> Incrementing the reference count and by this mapping the page into >> some other address space is illegal and corrupts the internal state >> tracking of TTM. > > See this comment in the source code as well: > >         /* Don't set the __GFP_COMP flag for higher order allocations. >          * Mapping pages directly into an userspace process and calling >          * put_page() on a TTM allocated page is illegal. >          */ > > I have absolutely no idea how somebody had the idea he could do this. I saw this comment, but it doesn't make sense because it doesn't explain why it's illegal. Hence it looks like a bogus comment since the refcouting certainly works, at least to a some degree because I haven't noticed any problems in practice, maybe by luck :) I'll try to dig out the older discussions, thank you for the quick reply! -- Best regards, Dmitry