Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device()

From: Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>
To: Joerg Roedel <joro@8bytes.org>
Cc: Stuart Yoder <stuyoder@gmail.com>,
	rafael@kernel.org, David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Ashok Raj <ashok.raj@intel.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v1 5/8] iommu/amd: Use iommu_attach/detach_device()
Date: Tue, 15 Feb 2022 10:37:53 -0400	[thread overview]
Message-ID: <20220215143753.GP4160@nvidia.com> (raw)
In-Reply-To: <YgtuJQhY8SNlv9/6@8bytes.org>

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
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