linux-kernel - Re: [PATCH 02/11] driver core: Set DMA ownership during driver bind/unbind

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Date:   Mon, 15 Nov 2021 11:56:13 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Diana Craciun <diana.craciun@....nxp.com>
Subject: Re: [PATCH 02/11] driver core: Set DMA ownership during driver
 bind/unbind

On Mon, Nov 15, 2021 at 03:37:18PM +0000, Robin Murphy wrote:

> IOMMUs, and possibly even fewer of them support VFIO, so I'm in full
> agreement with Greg and Christoph that this absolutely warrants being scoped
> per-bus. I mean, we literally already have infrastructure to prevent drivers
> binding if the IOMMU/DMA configuration is broken or not ready yet; why would
> we want a totally different mechanism to prevent driver binding when the
> only difference is that that configuration *is* ready and working to the
> point that someone's already claimed it for other purposes?

I see, that does make sense

I see these implementations:

drivers/amba/bus.c:     .dma_configure  = platform_dma_configure,
drivers/base/platform.c:        .dma_configure  = platform_dma_configure,
drivers/bus/fsl-mc/fsl-mc-bus.c:        .dma_configure  = fsl_mc_dma_configure,
drivers/pci/pci-driver.c:       .dma_configure  = pci_dma_configure,
drivers/gpu/host1x/bus.c:       .dma_configure = host1x_dma_configure,

Other than host1x they all work with VFIO.

Also, there is no bus->dma_unconfigure() which would be needed to
restore the device as well.

So, would you rather see duplicated code into the 4 drivers, and a new
bus op to 'unconfigure dma'

Or, a 'dev_configure_dma()' function that is roughly:

        if (dev->bus->dma_configure) {
                ret = dev->bus->dma_configure(dev);
                if (ret)
                        return ret;
                if (!drv->suppress_auto_claim_dma_owner) {
                       ret = iommu_device_set_dma_owner(dev, DMA_OWNER_KERNEL,
                                                        NULL);
                       if (ret)
                               ret;
                }
         }

And a pair'd undo.

This is nice because we can enforce dev->bus->dma_configure when doing
a user bind so everything holds together safely without relying on
each bus_type to properly implement security.

Jason