From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver Date: Wed, 19 Jul 2017 12:25:24 +0100 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-Transfer-Encoding: 7bit Cc: kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Scott Branden , Linux Kernel , Linux IOMMU , BCM Kernel Feedback , Linux ARM Kernel To: Anup Patel Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: kvm.vger.kernel.org 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