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

From: Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	linux-kernel@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH 2/5] iommu: Add blocking_domain_ops field in iommu_ops
Date: Mon, 16 May 2022 10:57:41 -0300	[thread overview]
Message-ID: <20220516135741.GV1343366@nvidia.com> (raw)
In-Reply-To: <8a0fc6cf-f46e-f17e-2b76-099ada1683c3@arm.com>

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
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