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Date:   Fri, 7 May 2021 15:15:11 -0700
From:   Jacob Pan <jacob.jun.pan@linux.intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "Raj, Ashok" <ashok.raj@intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        Auger Eric <eric.auger@redhat.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" <iommu@lists.linux-foundation.org>,
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        Jonathan Corbet <corbet@lwn.net>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>, jacob.jun.pan@linux.intel.com
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
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Hi Jason,

On Fri, 7 May 2021 16:28:10 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:

> > The unanswered question is how do we plumb from vIOMMU without a custom
> > allocator to get a system wide PASID?   
> 
> PASID allocation is part of the iommu driver, it really shouldn't be
> global.
> 
In the current code, the pluggable custom allocator *is* part of the iommu
vendor driver. If it decides the allocation is global then it should be
suitable for the platform since there will never be a VT-d IOMMU on another
vendor's platform.

It is true that the default allocator is global which suites the current
needs. I am just wondering if we are solving a problem does not exist yet.

> When the architecture code goes to allocate a single PASID for the
> mm_struct it should flag that allocation request with a 'must work for
> all RIDs flag' and the iommu driver should take care of it. That might
> mean the iommu driver consults a global static xarray, or maybe it
> does a hypercall, but it should be done through that API, not a side
> care global singleton.
Why do we need to flag the allocation every time if on a platform *every*
PASID can potentially be global? At the time of allocation, we don't know
if the PASID will be used for a shared (ENQCMD) or a dedicated workqueue.

Thanks,

Jacob