linux-kernel - Re: [RFC PATCH v2] vfio: Include No-IOMMU mode

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Date:	Thu, 15 Oct 2015 15:40:25 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	avi@...lladb.com, avi@...udius-systems.com, gleb@...lladb.com,
	corbet@....net, bruce.richardson@...el.com, mst@...hat.com,
	linux-kernel@...r.kernel.org, alexander.duyck@...il.com,
	gleb@...udius-systems.com, vladz@...udius-systems.com,
	iommu@...ts.linux-foundation.org, hjk@...sjkoch.de,
	gregkh@...uxfoundation.org
Subject: Re: [RFC PATCH v2] vfio: Include No-IOMMU mode

On Wed, 14 Oct 2015 15:51:18 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> There is really no way to safely give a user full access to a DMA
> capable device without an IOMMU to protect the host system.  There is
> also no way to provide DMA translation, for use cases such as device
> assignment to virtual machines.  However, there are still those users
> that want userspace drivers even under those conditions.  The UIO
> driver exists for this use case, but does not provide the degree of
> device access and programming that VFIO has.  In an effort to avoid
> code duplication, this introduces a No-IOMMU mode for VFIO.
> 
> This mode requires building VFIO with CONFIG_VFIO_NOIOMMU and enabling
> the "enable_unsafe_noiommu_mode" option on the vfio driver.  This
> should make it very clear that this mode is not safe.  Additionally,
> CAP_SYS_RAWIO privileges are necessary to work with groups and
> containers using this mode.  Groups making use of this support are
> named /dev/vfio/noiommu-$GROUP and can only make use of the special
> VFIO_NOIOMMU_IOMMU for the container.  Use of this mode, specifically
> binding a device without a native IOMMU group to a VFIO bus driver
> will taint the kernel and should therefore not be considered
> supported.  This patch includes no-iommu support for the vfio-pci bus
> driver only.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>

The concept looks good.

I am trying it now to see how well this works for the use case of DPDK
in a VM.

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