Date:   Mon, 15 Nov 2021 18:35:37 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        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>,
        linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Will Deacon <will@...nel.org>,
        Diana Craciun <diana.craciun@....nxp.com>
Subject: Re: [PATCH 02/11] driver core: Set DMA ownership during driver
 bind/unbind

On 2021-11-15 15:56, Jason Gunthorpe via iommu wrote:
> 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.

Not if we reduce the notion of "ownership" down to 
"dev->iommu_group->domain != dev->iommu_group->default_domain", which 
I'm becoming increasingly convinced is all we actually need here.

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

The .dma_configure flow is unavoidably a bit boilerplatey already, so 
personally I'd go for having the implementations call back into a common 
check, similarly to their current flow. That also leaves room for the 
bus code to further refine the outcome based on what it might know, 
which I can particularly imagine for cleverer buses like fsl-mc and 
host1x which can have lots of inside knowledge about how their devices 
may interact.

Robin.

> 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
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