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

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Date:   Thu, 9 Dec 2021 10:21:38 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com,
        "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

On 12/9/21 9:56 AM, Tian, Kevin wrote:
>> 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.

Yes, exactly. Imagining in the VM guest environment, do we require a
vIOMMU for this functionality? vIOMMU is not performance friendly if we
put aside the security considerations.

Best regards,
baolu