From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: [PATCH v6 4/9] vring: Introduce vring_use_dma_api() Date: Mon, 1 Feb 2016 10:00:54 -0800 Message-ID: <336edab2e5b4825abe899e10aa4368770362daf9.1454349471.git.luto@kernel.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: In-Reply-To: References: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: Joerg Roedel , KVM , linux-s390 , Benjamin Herrenschmidt , Stefano Stabellini , Sebastian Ott , "linux-kernel@vger.kernel.org" , Christoph Hellwig , Christian Borntraeger , David Vrabel , Andy Lutomirski , xen-devel@lists.xenproject.org, sparclinux@vger.kernel.org, Paolo Bonzini , Linux Virtualization , David Woodhouse , "David S. Miller" , Martin Schwidefsky List-Id: virtualization@lists.linuxfoundation.org This is a kludge, but no one has come up with a a better idea yet. We'll introduce DMA API support guarded by vring_use_dma_api(). Eventually we may be able to return true on more and more systems, and hopefully we can get rid of vring_use_dma_api() entirely some day. Signed-off-by: Andy Lutomirski --- drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index e12e385f7ac3..b78aeeb3f6be 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -104,6 +104,30 @@ struct vring_virtqueue { #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq) +/* + * The interaction between virtio and a possible IOMMU is a mess. + * + * On most systems with virtio, physical addresses match bus addresses, + * and it doesn't particularly matter whether we use the DMA API. + * + * On some systems, including Xen and any system with a physical device + * that speaks virtio behind a physical IOMMU, we must use the DMA API + * for virtio DMA to work at all. + * + * On other systems, including SPARC and PPC64, virtio-pci devices are + * enumerated as though they are behind an IOMMU, but the virtio host + * ignores the IOMMU, so we must either pretend that the IOMMU isn't + * there or somehow map everything as the identity. + * + * For the time being, we preserve historic behavior and bypass the DMA + * API. + */ + +static bool vring_use_dma_api(const struct vring_virtqueue *vq) +{ + return false; +} + static struct vring_desc *alloc_indirect(struct virtqueue *_vq, unsigned int total_sg, gfp_t gfp) { -- 2.5.0