linux-kernel - Re: [PATCH 1/4] ioasid: Reserve a global PASID for in-kernel DMA

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Date:   Fri, 10 Dec 2021 09:06:24 +0000
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Barry Song <21cnbao@...il.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/4] ioasid: Reserve a global PASID for in-kernel DMA

On Thu, Dec 09, 2021 at 10:14:04AM -0800, Jacob Pan wrote:
> > This looks like we're just one step away from device drivers needing
> > multiple PASIDs for kernel DMA so I'm trying to figure out how to evolve
> > the API towards that. It's probably as simple as keeping a kernel IOASID
> > set at first, but then we'll probably want to optimize by having multiple
> > overlapping sets for each device driver (all separate from the SVA set).
> Sounds reasonable to start with a kernel set for in-kernel DMA once we need
> multiple ones. But I am not sure what *overlapping* sets mean here, could
> you explain?

Given that each device uses a separate PASID table, we could allocate the
same set of PASID values for different device drivers. We just need to
make sure that those values are different from PASIDs allocated for user
SVA.

Thanks,
Jean