Re: [PATCH v2 1/1] iommu/vt-d: Fix RID2PASID setup failure

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2022/6/22 17:09, Ethan Zhao wrote:
> 
> 在 2022/6/22 12:41, Lu Baolu 写道:
>> The IOMMU driver shares the pasid table for PCI alias devices. When the
>> RID2PASID entry of the shared pasid table has been filled by the first
>> device, the subsequent devices will encounter the "DMAR: Setup RID2PASID
>> failed" failure as the pasid entry has already been marked as present. As
>> the result, the IOMMU probing process will be aborted.
>>
>> This fixes it by skipping RID2PASID setting if the pasid entry has been
>> populated. This works because the IOMMU core ensures that only the same
>> IOMMU domain can be attached to all PCI alias devices at the same time.
>> Therefore the subsequent devices just try to setup the RID2PASID entry
>> with the same domain, which is negligible. This also adds domain validity
>> checks for more confidence anyway.
>>
>> Fixes: ef848b7e5a6a0 ("iommu/vt-d: Setup pasid entry for RID2PASID 
>> support")
>> Reported-by: Chenyi Qiang <chenyi.qiang@intel.com>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>>   drivers/iommu/intel/pasid.c | 22 ++++++++++++++++------
>>   1 file changed, 16 insertions(+), 6 deletions(-)
>>
>> Change log:
>> v2:
>>   - Add domain validity check in RID2PASID entry setup.
>>
>> diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
>> index cb4c1d0cf25c..4f3525f3346f 100644
>> --- a/drivers/iommu/intel/pasid.c
>> +++ b/drivers/iommu/intel/pasid.c
>> @@ -575,6 +575,19 @@ static inline int pasid_enable_wpe(struct 
>> pasid_entry *pte)
>>       return 0;
>>   };
>> +/*
>> + * Return true if @pasid is RID2PASID and the domain @did has already
>> + * been setup to the @pte. Otherwise, return false. PCI alias devices
>> + * probably share the single RID2PASID pasid entry in the shared pasid
>> + * table. It's reasonable that those devices try to set a share domain
>> + * in their probe paths.
>> + */
> 
> I am thinking about the counter-part, the intel_pasid_tear_down_entry(),
> 
> Multi devices share the same PASID entry, then one was detached from the 
> domain,
> 
> so the entry doesn't exist anymore, while another devices don't know 
> about the change,
> 
> and they are using the mapping, is it possible case ？shared thing, no 
> refer-counter,
> 
> am I missing something ?

No. You are right. When any alias device is hot-removed from the system,
the shared RID2PASID will be cleared without any notification to other
devices. Hence any DMAs from those devices are blocked.

We still have a lot to do for sharing pasid table among alias devices.
Before we arrive there, let's remove it for now.

Best regards,
baolu
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