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

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Date:   Fri, 10 Dec 2021 08:31:09 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        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 Fri, Dec 10, 2021 at 09:06:24AM +0000, Jean-Philippe Brucker wrote:
> 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.

Why does user SVA need global values anyhow?

Jason