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Thu, 9 Jun 2022 10:02:44 +0000 (UTC) Received: from sirius.home.kraxel.org (unknown [10.39.192.40]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 39DB8492C3B; Thu, 9 Jun 2022 10:02:44 +0000 (UTC) Received: by sirius.home.kraxel.org (Postfix, from userid 1000) id B33291800094; Thu, 9 Jun 2022 12:02:42 +0200 (CEST) Date: Thu, 9 Jun 2022 12:02:42 +0200 From: Gerd Hoffmann To: Arnout Engelen Cc: qemu-devel@nongnu.org Subject: Re: [PATCH] hw/usb/hcd-ehci: fix writeback order Message-ID: <20220609100242.kyv2kxlfsg3tm52y@sirius.home.kraxel.org> References: <20220508153222.3560803-1-arnout@bzzt.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220508153222.3560803-1-arnout@bzzt.net> X-Scanned-By: MIMEDefang 2.85 on 10.11.54.9 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kraxel@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Sun, May 08, 2022 at 05:32:22PM +0200, Arnout Engelen wrote: > The 'active' bit passes control over a qTD between the guest and the > controller: set to 1 by guest to enable execution by the controller, > and the controller sets it to '0' to hand back control to the guest. > > ehci_state_writeback write two dwords to main memory using DMA: > the third dword of the qTD (containing dt, total bytes to transfer, > cpage, cerr and status) and the fourth dword of the qTD (containing > the offset). > > This commit makes sure the fourth dword is written before the third, > avoiding a race condition where a new offset written into the qTD > by the guest after it observed the status going to go to '0' gets > overwritten by a 'late' DMA writeback of the previous offset. > > This race condition could lead to 'cpage out of range (5)' errors, > and reproduced by: > > ./qemu-system-x86_64 -enable-kvm -bios $SEABIOS/bios.bin -m 4096 -device usb-ehci -blockdev driver=file,read-only=on,filename=/home/aengelen/Downloads/openSUSE-Tumbleweed-DVD-i586-Snapshot20220428-Media.iso,node-name=iso -device usb-storage,drive=iso,bootindex=0 -chardev pipe,id=shell,path=/tmp/pipe -device virtio-serial -device virtconsole,chardev=shell -device virtio-rng-pci -serial mon:stdio -nographic > > (press a key, select 'Installation' (2), and accept the default > values. On my machine the 'cpage out of range' is reproduced while > loading the Linux Kernel about once per 7 attempts. With the fix in > this commit it no longer fails) > > This problem was previously reported as a seabios problem in > https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/OUTHT5ISSQJGXPNTUPY3O5E5EPZJCHM3/ > and as a nixos CI build failure in > https://github.com/NixOS/nixpkgs/issues/170803 > > Signed-off-by: Arnout Engelen Patch queued up. thanks, Gerd