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Subject: Re: [PATCH v3 7/7] iommu/vt-d: Disable multiple GPASID-dev bind
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>,
        iommu@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>
Cc:     Yi Liu <yi.l.liu@intel.com>, "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>
References: <1593617636-79385-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1593617636-79385-8-git-send-email-jacob.jun.pan@linux.intel.com>
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <67186ca8-6af2-7ef2-a238-49fcfa5bc398@redhat.com>
Date:   Thu, 2 Jul 2020 10:50:54 +0200
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Hi Jacob,

On 7/1/20 5:33 PM, 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.
> 
> Fixes: 56722a4398a30 ("iommu/vt-d: Add bind guest PASID support")
> Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
> Cc: Kevin Tian <kevin.tian@intel.com>
> Cc: Lu Baolu <baolu.lu@linux.intel.com>
> Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric
> ---
>  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 {
>