From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:40588 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751062AbdBDNNO (ORCPT ); Sat, 4 Feb 2017 08:13:14 -0500 Subject: Patch "Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"" has been added to the 4.9-stable tree To: mst@redhat.com, gregkh@linuxfoundation.org, marc.zyngier@arm.com, robin.murphy@arm.com, will.deacon@arm.com Cc: , From: Date: Sat, 04 Feb 2017 14:13:02 +0100 Message-ID: <148621398247242@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org List-ID: This is a note to let you know that I've just added the patch titled Revert "vring: Force use of DMA API for ARM-based systems with legacy devices" to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: revert-vring-force-use-of-dma-api-for-arm-based-systems-with-legacy-devices.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let know about it. >>From 0d5415b489f68b58e1983a53793d25d53098ed4b Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Fri, 3 Feb 2017 05:43:52 +0200 Subject: Revert "vring: Force use of DMA API for ARM-based systems with legacy devices" From: Michael S. Tsirkin commit 0d5415b489f68b58e1983a53793d25d53098ed4b upstream. This reverts commit c7070619f3408d9a0dffbed9149e6f00479cf43b. This has been shown to regress on some ARM systems: by forcing on DMA API usage for ARM systems, we have inadvertently kicked open a hornets' nest in terms of cache-coherency. Namely that unless the virtio device is explicitly described as capable of coherent DMA by firmware, the DMA APIs on ARM and other DT-based platforms will assume it is non-coherent. This turns out to cause a big problem for the likes of QEMU and kvmtool, which generate virtio-mmio devices in their guest DTs but neglect to add the often-overlooked "dma-coherent" property; as a result, we end up with the guest making non-cacheable accesses to the vring, the host doing so cacheably, both talking past each other and things going horribly wrong. We are working on a safer work-around. Fixes: c7070619f340 ("vring: Force use of DMA API for ARM-based systems with legacy devices") Reported-by: Robin Murphy Signed-off-by: Will Deacon Signed-off-by: Michael S. Tsirkin Acked-by: Marc Zyngier Signed-off-by: Greg Kroah-Hartman --- drivers/virtio/virtio_ring.c | 7 ------- 1 file changed, 7 deletions(-) --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -159,13 +159,6 @@ static bool vring_use_dma_api(struct vir if (xen_domain()) return true; - /* - * On ARM-based machines, the DMA ops will do the right thing, - * so always use them with legacy devices. - */ - if (IS_ENABLED(CONFIG_ARM) || IS_ENABLED(CONFIG_ARM64)) - return !virtio_has_feature(vdev, VIRTIO_F_VERSION_1); - return false; } Patches currently in stable-queue which might be from mst@redhat.com are queue-4.9/vhost-fix-initialization-for-vq-is_le.patch queue-4.9/revert-vring-force-use-of-dma-api-for-arm-based-systems-with-legacy-devices.patch