On Wed, Jul 19, 2017 at 05:01:11PM +0530, Anup Patel wrote: > On Wed, Jul 19, 2017 at 4:55 PM, Will Deacon <will.deacon@xxxxxxx> wrote: > > On Wed, Jul 19, 2017 at 04:53:04PM +0530, Anup Patel wrote: > >> On Wed, Jul 19, 2017 at 4:30 PM, Robin Murphy <robin.murphy@xxxxxxx> 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 <anup.patel@xxxxxxxxxxxx> > >> >> --- > >> >> 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! > > We are keeping disable_bypass = false. In other words, we > are using bypass mode for unmatched streams. The real > reason is limited number of SMRs due to which we choose > not to provide "iommus" DT attribute for certain devices. Understood, but that's not robust for SMMUv3 and we *really* shouldn't have a user ABI that changes behaviour based on a cmdline option. VFIO should be requesting its own identity mappings, if that's what you need. Will
