Re: [PATCH] iommu/arm-smmu: Return IOVA in iova_to_phys when SMMU is bypassed

[Will Deacon]

On Mon, Apr 24, 2017 at 09:23:16PM +0530, Sunil Kovvuri wrote:
> On Mon, Apr 24, 2017 at 8:14 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Mon, Apr 17, 2017 at 05:27:26PM +0530, sunil.kovvuri@xxxxxxxxx wrote:
> >> From: Sunil Goutham <sgoutham@xxxxxxxxxx>
> >>
> >> For software initiated address translation, when domain type is
> >> IOMMU_DOMAIN_IDENTITY i.e SMMU is bypassed, mimic HW behavior
> >> i.e return the same IOVA as translated address.
> >>
> >> This patch is an extension to Will Deacon's patchset
> >> "Implement SMMU passthrough using the default domain".
> >
> > Are you actually seeing an issue here? If so, why isn't SMMUv3 affected too?
> Yes and SMMUv3 should also be effected but as of now I don't see any use case.
> If needed, i can re-submit the patch with changes in SMMUv3 as well.

Yes, please.

> >> Signed-off-by: Sunil Goutham <sgoutham@xxxxxxxxxx>
> >> ---
> >>  drivers/iommu/arm-smmu.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> >> index 41afb07..2f4a130 100644
> >> --- a/drivers/iommu/arm-smmu.c
> >> +++ b/drivers/iommu/arm-smmu.c
> >> @@ -1405,6 +1405,9 @@ static phys_addr_t arm_smmu_iova_to_phys(struct iommu_domain *domain,
> >>       struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
> >>       struct io_pgtable_ops *ops= smmu_domain->pgtbl_ops;
> >>
> >> +     if (domain->type == IOMMU_DOMAIN_IDENTITY)
> >> +             return iova;
> >> +
> >>       if (!ops)
> >>               return 0;
> >
> > I'd have thought ops would be NULL, since arm_smmu_init_domain_context
> > doesn't allocate them for an identity domain.
> Yes ops is set to NULL.

Argh, sorry, I completely overlooked that we return 0 in that case, rather
than the iova.

> > I don't understand this patch. Please can you explain the problem more
> > clearly?
> AFAIK for any driver outside IOMMU there is only one way to identify
> if device is attached to
> IOMMU or not and that is by checking iommu_domain. And I don't think
> it would be appropriate
> for the driver to check domain->type before calling 'iommu_iova_to_phys()' API.
> 
> The difference between IOMMU disabled and IOMMU being in passthrough
> mode is that, in the
> later case device is still attached to default domain but in former's
> case it's NULL. So there is no
> way to differentiate for the external driver whether IOMMU is in
> passthrough mode or DMA mode.
> 
> And since ops is NULL in passthrough mode, 'iommu_iova_to_phys()' will
> return zero.
> 
> Use case for your reference
> https://lkml.org/lkml/2017/3/7/299
> This driver is for a NIC interface on platform which supports SMMUv2.

Blimey, that driver is horrible, but I take your point on the API. Please
repost, fixing SMMUv3 at the same time.

Will