From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932081AbcA2Cbt (ORCPT ); Thu, 28 Jan 2016 21:31:49 -0500 Received: from mail.kernel.org ([198.145.29.136]:57181 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755821AbcA2Cbl (ORCPT ); Thu, 28 Jan 2016 21:31:41 -0500 From: Andy Lutomirski To: "Michael S. Tsirkin" Cc: Benjamin Herrenschmidt , David Woodhouse , "linux-kernel@vger.kernel.org" , "David S. Miller" , sparclinux@vger.kernel.org, Joerg Roedel , Christian Borntraeger , Cornelia Huck , Sebastian Ott , Paolo Bonzini , Christoph Hellwig , KVM , Martin Schwidefsky , linux-s390 , Linux Virtualization , David Vrabel , Stefano Stabellini , xen-devel@lists.xenproject.org, Andy Lutomirski Subject: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Date: Thu, 28 Jan 2016 18:31:17 -0800 Message-Id: X-Mailer: git-send-email 2.5.0 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.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