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

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Date:   Tue, 15 Feb 2022 10:11:01 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Jason Gunthorpe <jgg@...dia.com>
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 Mon, Feb 14, 2022 at 11:00:59AM -0400, Jason Gunthorpe wrote:
> On Mon, Feb 14, 2022 at 03:23:07PM +0100, Joerg Roedel wrote:
> 
> > Device drivers calling into iommu_attach_device() is seldom a good
> > idea.  In this case the sound device has some generic hardware
> > interface so that an existing sound driver can be re-used. Making this
> > driver call iommu-specific functions for some devices is something hard
> > to justify.
> 
> Er, so this is transparent to the generic sound device? I guess
> something fixed up the dma_api on that device to keep working?

Right, this is completly transparent to the sound device. The IOMMU code
will not set dma_ops on the device because it uses a direct mapping and
so the standard implementation will be used.

> 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.

It was actually a requirement for this code that when it jumps in, the
DMA-API mappings stay live. And the reason a direct mapping is used at
all is that the page-table walker of the IOMMU is a two-dimensional
walker, which will treat the addresses found in the host page-tables as
IO-virtual an translates them through the underlying page-table. So to
use host-pagetables the underlying mapping must be direct mapped.

> I don't think it matters how big/small the group is, only that when we
> change the domain we know everything flowing through the domain is
> still happy.

Yes, that matters. The group size matters too for DMA-API performance.
If two devices compete for the same lock in the allocator and/or the
same cached magazines, things will slow down. That only matters for
high-throughput devices, but still...

Regards,

	Joerg