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

[Sunil Kovvuri]

On Mon, Apr 24, 2017 at 9:30 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> 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.

Sure, will re-submit the patch with SMMUv3 changes.

On a separate note, if you have time, I would definitely like to know
your feedback and what's horrible in that driver, probably in a different
email to keep that out of scope of this patch.

Thanks,
Sunil.