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

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Message-ID: <20250103153434.GC3816@willie-the-truck>
Date: Fri, 3 Jan 2025 15:34:35 +0000
From: Will Deacon <will@...nel.org>
To: Charan Teja Kalla <quic_charante@...cinc.com>
Cc: joro@...tes.org, robin.murphy@....com, jgg@...pe.ca,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] iommu: fix wrong DMA ops for iommu device

(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@...cinc.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