linux-kernel - Re: [PATCH 3/4] iommu/sva: Support reservation of global PASIDs

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Date:   Mon, 6 Mar 2023 09:01:32 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>
Subject: Re: [PATCH 3/4] iommu/sva: Support reservation of global PASIDs

On Fri, Mar 03, 2023 at 01:47:53PM -0800, Jacob Pan wrote:
> Hi Kevin,
> 
> On Thu, 2 Mar 2023 09:43:03 +0000, "Tian, Kevin" <kevin.tian@...el.com>
> wrote:
> 
> > > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > Sent: Thursday, March 2, 2023 9:00 AM
> > > 
> > > Global PASID allocation is under IOMMU SVA code since it is the primary
> > > use case.  However, some architecture such as VT-d, global PASIDs are
> > > necessary for its internal use of DMA API with PASID.  
> > 
> > No, global PASID is not a VT-d restriction. It's from ENQCMD/S hence a
> > device requirement.
> I meant VT-d based platforms, it is kind of intertwined in that ENQCMDS
> does not restrict RIDPASID!=DMA PASID, vt-d does. Without this
> restriction, there wouldn't be a need for this patch. Let me reword.

No, Kevin is right, there is nothing about VT-d that needs global
PASID values.

The driver should be managing RID2PASID itself to avoid conflicting
with any in-use PASID, either by changing RID2PASID on demand or by
setting it to a value that is not part of the PASID number space, eg
we can make 0 entirely invalid, or the driver can reduce max_pasid of
the devices it controls and use PASID_MAX.

Jason