From: Michael S. Tsirkin Date: Fri, 3 Feb 2017 03:43:52 +0000 (+0200) Subject: Revert "vring: Force use of DMA API for ARM-based systems with legacy devices" X-Git-Tag: v4.14-rc1~1589^2~2 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=0d5415b489f68b58e1983a53793d25d53098ed4b;p=platform%2Fkernel%2Flinux-rpi.git Revert "vring: Force use of DMA API for ARM-based systems with legacy devices" 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 Cc: Signed-off-by: Will Deacon Signed-off-by: Michael S. Tsirkin Acked-by: Marc Zyngier --- diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 7e38ed7..409aeaa4 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -159,13 +159,6 @@ static bool vring_use_dma_api(struct virtio_device *vdev) 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; }