Re: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in debugfs

From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>, Baolu Lu <baolu.lu@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.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 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in debugfs
Date: Tue, 31 May 2022 16:01:04 +0100	[thread overview]
Message-ID: <a7d6d830-cb06-e0d7-0688-028f9af900e5@arm.com> (raw)
In-Reply-To: <20220531145301.GE1343366@nvidia.com>

On 2022-05-31 15:53, Jason Gunthorpe wrote:
> On Tue, May 31, 2022 at 10:11:18PM +0800, Baolu Lu wrote:
>> On 2022/5/31 21:10, Jason Gunthorpe wrote:
>>> On Tue, May 31, 2022 at 11:02:06AM +0800, Baolu Lu wrote:
>>>
>>>> For case 2, it is a bit weird. I tried to add a rwsem lock to make the
>>>> iommu_unmap() and dumping tables in debugfs exclusive. This does not
>>>> work because debugfs may depend on the DMA of the devices to work. It
>>>> seems that what we can do is to allow this race, but when we traverse
>>>> the page table in debugfs, we will check the validity of the physical
>>>> address retrieved from the page table entry. Then, the worst case is to
>>>> print some useless information.
>>>
>>> Sounds horrible, don't you have locking around the IOPTEs of some
>>> kind? How does updating them work reliably?
>>
>> There's no locking around updating the IOPTEs. The basic assumption is
>> that at any time, there's only a single thread manipulating the mappings
>> of the range specified in iommu_map/unmap() APIs. Therefore, the race
>> only exists when multiple ranges share some high-level IOPTEs. The IOMMU
>> driver updates those IOPTEs using the compare-and-exchange atomic
>> operation.
> 
> Oh? Did I miss where that was documented as part of the iommu API?
> 
> Daniel posted patches for VFIO to multi-thread iommu_domin mapping.
> 
> iommufd goes out of its way to avoid this kind of serialization so
> that userspace can parallel map IOVA.
> 
> I think if this is the requirement then the iommu API needs to
> provide a lock around the domain for the driver..

Eww, no, we can't kill performance by forcing serialisation on the 
entire API just for one silly driver-internal debugfs corner :(

Robin.
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