linux-kernel - Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach

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Date:   Tue, 15 Feb 2022 10:37:53 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device()

On Tue, Feb 15, 2022 at 10:11:01AM +0100, Joerg Roedel wrote:

> > But, then, the requirement is that nobody is using the dma API when we
> > make this change?
> 
> That is the tricky part. DMA-API keeps working after the change is made,
> because the new domain is also direct mapped. The new domain just has
> the ability to assign host page-tables to device PASIDs, so that DMA
> requests with a PASID TLP will be remapped.

From, what you've described this is also a good use case for the
replace_group idea..

Using Lu's series we'd:
 - Require the group to be in DMA-API mode. We know this is the case
   because of how we got here from probe()
 - Use the 'replace group's domain' API to switch the group from one
   identity translation domain to another identity translation domain
 - Rely on the DMA-API refcount to block VFIO/etc instead of checking
   for the default domain
 - Restore the default domain when all the DMA-API mode drivers unprobe

That is, if Robin's idea to just get the right domain in the first
place doesn't work.

Anyhow, given all this I think this patch is not OK as is.

Thanks,
Jason