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Date:   Tue, 13 Apr 2021 17:09:47 -0700
From:   Jacob Pan <jacob.jun.pan@linux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org, Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Yi Liu <yi.l.liu@intel.com>, Raj Ashok <ashok.raj@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Dave Jiang <dave.jiang@intel.com>, wangzhou1@hisilicon.com,
        zhangfei.gao@linaro.org, vkoul@kernel.org,
        jacob.jun.pan@linux.intel.com
Subject: Re: [PATCH 2/2] iommu/sva: Remove mm parameter from SVA bind API
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In-Reply-To: <20210409110305.6b0471d9@jacob-builder>
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Hi Jean,

On Fri, 9 Apr 2021 11:03:05 -0700, Jacob Pan
<jacob.jun.pan@linux.intel.com> wrote:

> > problems:
> > 
> > * We don't have a use-case for binding the mm of a remote process (and
> >   it's supposedly difficult for device drivers to do it securely). So
> > OK, we remove the mm argument from iommu_sva_bind_device() and use the
> >   current mm. But the IOMMU driver isn't going to do
> > get_task_mm(current) every time it needs the mm being bound, it will
> > take it from iommu_sva_bind_device(). Likewise iommu_sva_alloc_pasid()
> > shouldn't need to bother with get_task_mm().
> > 
> > * cgroup accounting for IOASIDs needs to be on the current task.
> > Removing the mm parameter from iommu_sva_alloc_pasid() doesn't help
> > with that. Sure it indicates that iommu_sva_alloc_pasid() needs a
> > specific task context but that's only for cgroup purpose, and I'd
> > rather pass the cgroup down from iommu_sva_bind_device() anyway (but am
> > fine with keeping it within ioasid_alloc() for now). Plus it's an
> > internal helper, easy for us to check that the callers are doing the
> > right thing. 
> With the above split, we really just have one allocation function:
> ioasid_alloc(), so it can manage current cgroup accounting within. Would
> this work?
After a few attempts, I don't think the split can work better. I will
restore the mm parameter and add a warning if mm != current->mm.

Thanks,

Jacob