linux-kernel - Re: [PATCH v4 1/6] iommu: Add a per domain PASID for DMA API

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Message-ID: <20220530122247.GY1343366@nvidia.com>
Date:   Mon, 30 May 2022 09:22:47 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>, dmaengine@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>, vkoul@...nel.org,
        robin.murphy@....com, will@...nel.org, Yi Liu <yi.l.liu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH v4 1/6] iommu: Add a per domain PASID for DMA API

On Tue, May 24, 2022 at 08:17:27AM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Tue, 24 May 2022 10:50:34 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Wed, May 18, 2022 at 11:21:15AM -0700, Jacob Pan wrote:
> > > DMA requests tagged with PASID can target individual IOMMU domains.
> > > Introduce a domain-wide PASID for DMA API, it will be used on the same
> > > mapping as legacy DMA without PASID. Let it be IOVA or PA in case of
> > > identity domain.  
> > 
> > Huh? I can't understand what this is trying to say or why this patch
> > makes sense.
> > 
> > We really should not have pasid's like this attached to the domains..
> > 
> This is the same "DMA API global PASID" you reviewed in v3, I just
> singled it out as a standalone patch and renamed it. Here is your previous
> review comment.
> 
> > +++ b/include/linux/iommu.h
> > @@ -105,6 +105,8 @@ struct iommu_domain {
> >  	enum iommu_page_response_code (*iopf_handler)(struct iommu_fault *fault,
> >  						      void *data);
> >  	void *fault_data;
> > +	ioasid_t pasid;		/* Used for DMA requests with PASID */
> > +	atomic_t pasid_users;  
> 
> These are poorly named, this is really the DMA API global PASID and
> shouldn't be used for other things.
> 
> 
> 
> Perhaps I misunderstood, do you mind explaining more?

You still haven't really explained what this is for in this patch,
maybe it just needs a better commit message, or maybe something is
wrong.

I keep saying the DMA API usage is not special, so why do we need to
create a new global pasid and refcount? Realistically this is only
going to be used by IDXD, why can't we just allocate a PASID and
return it to the driver every time a driver asks for DMA API on PASI
mode? Why does the core need to do anything special?

Jason