Re: [PATCH 2/5] iommu: Add blocking_domain_ops field in iommu_ops

Jason Gunthorpe
2022-05-16T13:58:18Z

On Mon, May 16, 2022 at 12:22:08PM +0100, Robin Murphy wrote:
> On 2022-05-16 02:57, Lu Baolu wrote:
> > Each IOMMU driver must provide a blocking domain ops. If the hardware
> > supports detaching domain from device, setting blocking domain equals
> > detaching the existing domain from the deivce. Otherwise, an UNMANAGED
> > domain without any mapping will be used instead.
>
> Unfortunately that's backwards - most of the implementations of .detach_dev
> are disabling translation entirely, meaning the device ends up effectively
> in passthrough rather than blocked.

Ideally we'd convert the detach_dev of every driver into either
a blocking or identity domain. The trick is knowing which is which..

Guessing going down the list:
apple dart - blocking, detach_dev calls apple_dart_hw_disable_dma() same as
IOMMU_DOMAIN_BLOCKED
[I wonder if this drive ris wrong in other ways though because
I dont see a remove_streams in attach_dev]
exynos - this seems to disable the 'sysmmu' so I'm guessing this is
identity
iommu-vmsa - Comment says 'disable mmu translaction' so I'm guessing
this is idenity
mkt_v1 - Code looks similar to mkt, which is probably identity.
rkt - No idea
sprd - No idea
sun50i - This driver confusingly treats identity the same as
unmanaged, seems wrong, no idea.
amd - Not sure, clear_dte_entry() seems to set translation on but points
the PTE to 0 ? Based on the spec table 8 I would have expected
TV to be clear which would be blocking. Maybe a bug??
arm smmu qcomm - not sure
intel - blocking

These doesn't support default domains, so detach_dev should return
back to DMA API ownership, which is either identity or something weird:
fsl_pamu - identity due to the PPC use of dma direct
msm
mkt
omap
s390 - platform DMA ops
terga-gart - Usually something called a GART would be 0 length once
disabled, guessing blocking?
tegra-smmu

So, the approach here should be to go driver by driver and convert
detach_dev to either identity, blocking or just delete it entirely,
excluding the above 7 that don't support default domains. And get acks
from the driver owners.

> Conversely, at least arm-smmu and arm-smmu-v3 could implement
> IOMMU_DOMAIN_BLOCKED properly with fault-type S2CRs and STEs
> respectively, it just needs a bit of wiring up.

Given that vfio now uses them it seems worthwhile to do..

Jason