Re: [PATCH RFC] iommu: fix wrong DMA ops for iommu device

Will Deacon
2025-01-03T15:34:40Z

(Please try not to top-post as it makes the thread hard to follow)

On Thu, Jan 02, 2025 at 04:09:46PM +0530, Charan Teja Kalla wrote:
> On 12/13/2024 8:34 PM, Charan Teja Kalla wrote:
> > Race between smmu device probe and iommu device probe is resulting into
> > wrong DMA ops used for the device.
> >
> > bus_iommu_probe(P1) of_iommu_configure(P2)
> > (from iommu_device_register) (from insmod of a driver)
> > -------------------------- --------------------------
> > 1) probe_iommu_group()
> > (Fills just dev->iommu_group
> > under iommu_probe_device_lock)
> > 2) iommu_probe_device():
> > (acquires the iommu_probe_device_lock,
> > but since dev->iommu_group is already
> > set, it just returns without
> > allocating the domain.)
> >
> > 3) of_dma_configure_id()->
> > arch_setup_dma_ops() gets called for
> > this device, but returns with the
> > error message:
> > "Failed to set up IOMMU for device;
> > retaining platform DMA ops"
> >
> > 4) device probe is succeeded where it
> > can now setup iommu mappings using
> > wrong ->dma_ops
> >
> > 5) domain will be allocated later
> > and fills iommu_group->domain.
> >
> > Step 3) and 4) denies iommu device from using 'iommu_dma_ops'.
> >
> > This issue does exists on 6.6 because of commit f188056352bc ("iommu:
> > Avoid locking/unlocking for iommu_probe_device()") without which 2)
> > returns only after filling the domain under group->mutex, thus no issue.
> >
> > With commit b5c58b2fdc42("dma-mapping: direct calls for dma-iommu"),
> > ->iommu_dma_ops is removed altogether and iommu api are directly being
> > called under "use_dma_iommu(): return dev->dma_iommu". Again,
> > ->dma_iommu flag is set only after domain allocation. So, If
> > there is a sufficient delay between steps 4) and 5), issue is still
> > exists but that seems very narrow to me.
> >
> > I am thinking, setup of the domain before returning from step2) is the
> > way to fix this issue. Can you please help if you have any suggestions?
> >
> > Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
> > ---
> > drivers/iommu/iommu.c | 26 +++++++++++++++++++++++++-
> > 1 file changed, 25 insertions(+), 1 deletion(-)
>
> Just a quick ask if you have any comments for the below. BTW, in
> internal tests, when we revert the commit f188056352bc ("iommu: Avoid
> locking/unlocking for iommu_probe_device()", it is helping.
>
> I can share more details If the below is not sufficient.
>
> Thanks in advance...

Can you reproduce the issue on 6.13-rc5? We bodged some of the probe
code recently:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=229e6ee43d2a160a1592b83aad620d6027084aad

and a better set of fixes are queued in -next.

Will