From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Date: Thu, 28 Jan 2016 18:31:17 -0800 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1aOyqd-00012T-6e for xen-devel@lists.xenproject.org; Fri, 29 Jan 2016 02:31:43 +0000 In-Reply-To: In-Reply-To: References: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: "Michael S. Tsirkin" Cc: Cornelia Huck , 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: xen-devel@lists.xenproject.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..4b8dab4960bb 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -25,6 +25,30 @@ #include #include +/* + * 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 DMI API. + * + * On some sytems, 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 preseve historic behavior and bypass the DMA + * API. + */ + +static bool vring_use_dma_api(void) +{ + return false; +} + #ifdef DEBUG /* For development, we want to crash whenever the ring is screwed. */ #define BAD_RING(_vq, fmt, args...) \ -- 2.5.0