Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754232AbdGSLZW (ORCPT ); Wed, 19 Jul 2017 07:25:22 -0400 Received: from foss.arm.com ([217.140.101.70]:38332 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754176AbdGSLZU (ORCPT ); Wed, 19 Jul 2017 07:25:20 -0400 Date: Wed, 19 Jul 2017 12:25:24 +0100 From: Will Deacon To: Anup Patel Cc: Robin Murphy , Joerg Roedel , Baptiste Reynal , Alex Williamson , Scott Branden , Linux Kernel , Linux ARM Kernel , Linux IOMMU , kvm@vger.kernel.org, BCM Kernel Feedback Subject: Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver Message-ID: <20170719112524.GF13642@arm.com> References: <1500456838-18405-1-git-send-email-anup.patel@broadcom.com> <1500456838-18405-4-git-send-email-anup.patel@broadcom.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1889 Lines: 43 On Wed, Jul 19, 2017 at 04:53:04PM +0530, Anup Patel wrote: > On Wed, Jul 19, 2017 at 4:30 PM, Robin Murphy wrote: > > On 19/07/17 10:33, Anup Patel wrote: > >> The ARM SMMUv3 support bypassing transactions for which domain > >> is not configured. The patch adds corresponding IOMMU capability > >> to advertise this fact. > >> > >> Signed-off-by: Anup Patel > >> --- > >> drivers/iommu/arm-smmu-v3.c | 2 ++ > >> 1 file changed, 2 insertions(+) > >> > >> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c > >> index 568c400..a6c7f66 100644 > >> --- a/drivers/iommu/arm-smmu-v3.c > >> +++ b/drivers/iommu/arm-smmu-v3.c > >> @@ -1423,6 +1423,8 @@ static bool arm_smmu_capable(enum iommu_cap cap) > >> return true; > >> case IOMMU_CAP_NOEXEC: > >> return true; > >> + case IOMMU_CAP_BYPASS: > >> + return true; > > > > And this is never true. If Linux knows a device masters through the > > SMMU, it will always have a default domain of some sort (either identity > > or DMA ops). If Linux doesn't know, then it won't have been able to > > initialise the stream table for the relevant stream IDs, thus any > > 'bypass' DMA is going to raise C_BAD_STE. SMMUv3 can effectively only > > bypass unknown stream IDs if disabled entirely. > > What if we don't want to use IOMMU for certain device and > due to this we never provide "iommus" DT attribute in the > device DT node. Further, we want to access device without > "iommus" DT attribute from user-space using VFIO no-IOMMU. Wait, you want to pass a device through to userspace but you don't want to use the IOMMU? Why not? If you describe the SMMU in firmware with only a partial topology description, then you will run into problems with unknown masters trying to perform DMA. That's the IOMMU doing its job! Will