Re: [PATCH 7/7] iommu/vt-d: Disable multiple GPASID-dev bind

[Lu Baolu]

On 2020/6/23 23:43, Jacob Pan wrote:
> For the unlikely use case where multiple aux domains from the same pdev
> are attached to a single guest and then bound to a single process
> (thus same PASID) within that guest, we cannot easily support this case
> by refcounting the number of users. As there is only one SL page table
> per PASID while we have multiple aux domains thus multiple SL page tables
> for the same PASID.
>
> Extra unbinding guest PASID can happen due to race between normal and
> exception cases. Termination of one aux domain may affect others unless
> we actively track and switch aux domains to ensure the validity of SL
> page tables and TLB states in the shared PASID entry.
>
> Support for sharing second level PGDs across domains can reduce the
> complexity but this is not available due to the limitations on VFIO
> container architecture. We can revisit this decision once sharing PGDs
> are available.
>
> Overall, the complexity and potential glitch do not warrant this unlikely
> use case thereby removed by this patch.
>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>

Fixes: 56722a4398a30 ("iommu/vt-d: Add bind guest PASID support")
Acked-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>

Best regards,
baolu

> ---
>   drivers/iommu/intel/svm.c | 22 +++++++++-------------
>   1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
> index 6c87c807a0ab..d386853121a2 100644
> --- a/drivers/iommu/intel/svm.c
> +++ b/drivers/iommu/intel/svm.c
> @@ -277,20 +277,16 @@ int intel_svm_bind_gpasid(struct iommu_domain *domain, struct device *dev,
>   			goto out;
>   		}
>   
> +		/*
> +		 * Do not allow multiple bindings of the same device-PASID since
> +		 * there is only one SL page tables per PASID. We may revisit
> +		 * once sharing PGD across domains are supported.
> +		 */
>   		for_each_svm_dev(sdev, svm, dev) {
> -			/*
> -			 * For devices with aux domains, we should allow
> -			 * multiple bind calls with the same PASID and pdev.
> -			 */
> -			if (iommu_dev_feature_enabled(dev,
> -						      IOMMU_DEV_FEAT_AUX)) {
> -				sdev->users++;
> -			} else {
> -				dev_warn_ratelimited(dev,
> -						     "Already bound with PASID %u\n",
> -						     svm->pasid);
> -				ret = -EBUSY;
> -			}
> +			dev_warn_ratelimited(dev,
> +					     "Already bound with PASID %u\n",
> +					     svm->pasid);
> +			ret = -EBUSY;
>   			goto out;
>   		}
>   	} else {