linux-kernel - RE: [PATCH v6 03/12] iommu: Add attach/detach_dev

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Date:   Wed, 11 May 2022 04:09:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jean-Philippe Brucker" <jean-philippe@...aro.org>
Subject: RE: [PATCH v6 03/12] iommu: Add attach/detach_dev_pasid domain ops

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, May 11, 2022 10:32 AM
> 
> On 2022/5/10 22:02, Jason Gunthorpe wrote:
> > On Tue, May 10, 2022 at 02:17:29PM +0800, Lu Baolu wrote:
> >
> >> This adds a pair of common domain ops for this purpose and adds
> helpers
> >> to attach/detach a domain to/from a {device, PASID}.
> >
> > I wonder if this should not have a detach op - after discussing with
> > Robin we can see that detach_dev is not used in updated
> > drivers. Instead attach_dev acts as 'set_domain'
> >
> > So, it would be more symmetrical if attaching a blocking_domain to the
> > PASID was the way to 'detach'.
> >
> > This could be made straightforward by following the sketch I showed to
> > have a static, global blocing_domain and providing a pointer to it in
> > struct iommu_ops
> >
> > Then 'detach pasid' is:
> >
> > iommu_ops->blocking_domain->ops->attach_dev_pasid(domain, dev,
> pasid);
> >
> > And we move away from the notion of 'detach' and in the direction that
> > everything continuously has a domain set. PASID would logically
> > default to blocking_domain, though we wouldn't track this anywhere.
> 
> I am not sure whether we still need to keep the blocking domain concept
> when we are entering the new PASID world. Please allow me to wait and
> listen to more opinions.
> 

I'm with Jason on this direction. In concept after a PASID is detached it's
essentially blocked. Implementation-wise it doesn't prevent the iommu
driver from marking the PASID entry as non-present as doing in this
series instead of actually pointing to the empty page table of the block
domain. But api-wise it does make the entire semantics more consistent.

Thanks
Kevin