linux-kernel - Re: [PATCH v6 1/4] iommu: Generalize default PCIe requester ID PASID

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Message-ID: <20230523082621.51f103cc@jacob-builder>
Date:   Tue, 23 May 2023 08:26:21 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        Jason Gunthorpe <jgg@...dia.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>, dmaengine@...r.kernel.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>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        narayan.ranganathan@...el.com, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v6 1/4] iommu: Generalize default PCIe requester ID
 PASID

Hi Jean,

On Tue, 23 May 2023 15:47:33 +0100, Jean-Philippe Brucker
<jean-philippe@...aro.org> wrote:

> Hi Jacob,
> 
> On Fri, May 19, 2023 at 01:32:20PM -0700, Jacob Pan wrote:
> > PCIe Process address space ID (PASID) is used to tag DMA traffic, it
> > provides finer grained isolation than requester ID (RID).
> > 
> > For each RID, 0 is as a special PASID for the legacy DMA (without
> > PASID), thus RID_PASID. This is universal across all architectures,
> > therefore warranted to be declared in the common header.
> > Noting that VT-d could support none-zero RID_PASID, but currently not
> > used.
> > 
> > By having a common RID_PASID, we can avoid conflicts between different
> > use cases in the generic code. e.g. SVA and DMA API with PASIDs.
> > 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > ---
> > v6:
> >    - let SMMU code use the common RID_PASID macro
> > ---
> >  .../iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c   |  2 +-
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 10 ++++----
> >  drivers/iommu/intel/iommu.c                   | 24 +++++++++----------
> >  drivers/iommu/intel/pasid.c                   |  2 +-
> >  drivers/iommu/intel/pasid.h                   |  1 -
> >  include/linux/iommu.h                         |  1 +
> >  6 files changed, 20 insertions(+), 20 deletions(-)
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
> > b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c index
> > a5a63b1c947e..160b31e6239d 100644 ---
> > a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c +++
> > b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c @@ -80,7 +80,7 @@
> > arm_smmu_share_asid(struct mm_struct *mm, u16 asid)
> >  	 * be some overlap between use of both ASIDs, until we
> > invalidate the
> >  	 * TLB.
> >  	 */
> > -	arm_smmu_write_ctx_desc(smmu_domain, 0, cd);
> > +	arm_smmu_write_ctx_desc(smmu_domain, IOMMU_DEF_RID_PASID, cd);
> >  
> 
> I agree with reserving 0 globally for non-PASID DMA, but could we call
> this something more generic, like IOMMU_NO_PASID?  The term "RID_PASID" is
> specific to VT-d and "RID" to PCI, so it looks confusing here (this driver
> also supports non-PCI). "NO_PASID" would be clearer to someone just trying
> to follow this driver code.
> 
Sounds good, it is for DMA w/o PASID.


Thanks,

Jacob