linux-kernel - RE: [PATCH 2/4] iommu: Add PASID support for DMA mapping API users

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Date:   Thu, 9 Dec 2021 01:56:37 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
CC:     "iommu@...ts.linux-foundation.org" <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>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, Barry Song <21cnbao@...il.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: RE: [PATCH 2/4] iommu: Add PASID support for DMA mapping API users

> From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Sent: Thursday, December 9, 2021 2:50 AM
> 
> > Can a device issue DMA requests with PASID even there's no system
> IOMMU
> > or the system IOMMU is disabled?
> >
> Good point.
> If IOMMU is not enabled, device cannot issue DMA requests with PASID. This
> API will not be available. Forgot to add dummy functions to the header.
> 

PASID is a PCI thing, not defined by IOMMU.

I think the key is physically if IOMMU is disabled, how will root complex
handle a PCI memory request including a PASID TLP prefix? Does it block 
such request due to no IOMMU to consume PASID or simply ignore PASID
and continue routing the request to the memory controller?

If block, then having an iommu interface makes sense.

If ignore, possibly a DMA API call makes more sense instead, implying that
this extension can be used even when iommu is disabled.

I think that is what Baolu wants to point out.

Thanks
Kevin