On Wed, Jul 19, 2017 at 5:03 PM, Will Deacon <will.deacon@xxxxxxx> wrote: > 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. Currently, the iommu_present() check in vfio_iommu_group_get() is preventing us allow no-IOMMU mode for certain devices. Is there a better replacement of iommu_present() check in vfio_iommu_group_get()? Regards, Anup
