Re: [RFC 05/20] vfio/pci: Register device to /dev/vfio/devices

From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: kvm@vger.kernel.org, jasowang@redhat.com, kwankhede@nvidia.com,
	hch@lst.de, jean-philippe@linaro.org, dave.jiang@intel.com,
	ashok.raj@intel.com, corbet@lwn.net, kevin.tian@intel.com,
	parav@mellanox.com, lkml@metux.net, david@gibson.dropbear.id.au,
	dwmw2@infradead.org, jun.j.tian@intel.com,
	linux-kernel@vger.kernel.org, lushenming@huawei.com,
	iommu@lists.linux-foundation.org, pbonzini@redhat.com,
	robin.murphy@arm.com
Subject: Re: [RFC 05/20] vfio/pci: Register device to /dev/vfio/devices
Date: Tue, 21 Sep 2021 15:09:29 -0600	[thread overview]
Message-ID: <20210921150929.5977702c.alex.williamson@redhat.com> (raw)
In-Reply-To: <20210921164001.GR327412@nvidia.com>

On Tue, 21 Sep 2021 13:40:01 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Sun, Sep 19, 2021 at 02:38:33PM +0800, Liu Yi L wrote:
> > This patch exposes the device-centric interface for vfio-pci devices. To
> > be compatiable with existing users, vfio-pci exposes both legacy group
> > interface and device-centric interface.
> > 
> > As explained in last patch, this change doesn't apply to devices which
> > cannot be forced to snoop cache by their upstream iommu. Such devices
> > are still expected to be opened via the legacy group interface.

This doesn't make much sense to me.  The previous patch indicates
there's work to be done in updating the kvm-vfio contract to understand
DMA coherency, so you're trying to limit use cases to those where the
IOMMU enforces coherency, but there's QEMU work to be done to support
the iommufd uAPI at all.  Isn't part of that work to understand how KVM
will be told about non-coherent devices rather than "meh, skip it in the
kernel"?  Also let's not forget that vfio is not only for KVM.

> > When the device is opened via /dev/vfio/devices, vfio-pci should prevent
> > the user from accessing the assigned device because the device is still
> > attached to the default domain which may allow user-initiated DMAs to
> > touch arbitrary place. The user access must be blocked until the device
> > is later bound to an iommufd (see patch 08). The binding acts as the
> > contract for putting the device in a security context which ensures user-
> > initiated DMAs via this device cannot harm the rest of the system.
> > 
> > This patch introduces a vdev->block_access flag for this purpose. It's set
> > when the device is opened via /dev/vfio/devices and cleared after binding
> > to iommufd succeeds. mmap and r/w handlers check this flag to decide whether
> > user access should be blocked or not.  
> 
> This should not be in vfio_pci.
> 
> AFAIK there is no condition where a vfio driver can work without being
> connected to some kind of iommu back end, so the core code should
> handle this interlock globally. A vfio driver's ops should not be
> callable until the iommu is connected.
> 
> The only vfio_pci patch in this series should be adding a new callback
> op to take in an iommufd and register the pci_device as a iommufd
> device.

Couldn't the same argument be made that registering a $bus device as an
iommufd device is a common interface that shouldn't be the
responsibility of the vfio device driver?  Is userspace opening the
non-group device anything more than a reservation of that device if
access is withheld until iommu isolation?  I also don't really want to
predict how ioctls might evolve to guess whether only blocking .read,
.write, and .mmap callbacks are sufficient.  Thanks,

Alex

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