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Date:   Tue, 30 Mar 2021 15:42:24 +0200
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Jacob Pan <jacob.jun.pan@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        iommu@lists.linux-foundation.org, cgroups@vger.kernel.org,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Raj Ashok <ashok.raj@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>, Yi Liu <yi.l.liu@intel.com>,
        Wu Hao <hao.wu@intel.com>, Dave Jiang <dave.jiang@intel.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
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On Tue, Mar 30, 2021 at 10:07:55AM -0300, Jason Gunthorpe wrote:
> On Fri, Mar 26, 2021 at 09:06:42AM +0100, Jean-Philippe Brucker wrote:
> 
> > It's not inconceivable to have a control queue doing DMA tagged with
> > PASID. The devices I know either use untagged DMA, or have a choice to use
> > a PASID.
> 
> I don't think we should encourage that. A PASID and all the related is
> so expensive compared to just doing normal untagged kernel DMA.

How is it expensive?  Low number of PASIDs, or slowing down DMA
transactions?  PASIDs aren't a scarce resource on Arm systems, they have
almost 1M unused PASIDs per VM.

Thanks,
Jean

> I assume HW has these features because virtualization use cases might
> use them, eg by using mdev to assign a command queue - then it would
> need be be contained by a PASID.
> 
> Jason