Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

Jerome Glisse
2018-09-07T18:04:20Z

On Fri, Sep 07, 2018 at 06:55:45PM +0100, Jean-Philippe Brucker wrote:
> On 07/09/2018 17:53, Jerome Glisse wrote:
> > So there is no reasons to do that under VFIO. Especialy as in your example
> > it is not a real user space device driver, the userspace portion only knows
> > about writting command into command buffer AFAICT.
> >
> > VFIO is for real userspace driver where interrupt, configurations, ... ie
> > all the driver is handled in userspace. This means that the userspace have
> > to be trusted as it could program the device to do DMA to anywhere (if
> > IOMMU is disabled at boot which is still the default configuration in the
> > kernel).
>
> If the IOMMU is disabled (not exactly a kernel default by the way, I
> think most IOMMU drivers enable it by default), your userspace driver
> can't bypass DMA isolation by accident. It just won't be allowed to
> access the device. VFIO requires an IOMMU unless the admin forces the
> NOIOMMU mode with the "enable_unsafe_noiommu_mode" module parameter, and
> the userspace explicitly asks for it with VFIO_NOIOMMU_IOMMU, which
> taints the kernel. Not for production. A normal userspace driver that
> uses VFIO can only do DMA to its own memory.
>

Didn't know about VFIO check, which is a sane thing. On Intel IOMMU
is disabled by default (see INTEL_IOMMU_DEFAULT_ON Kconfig option).
I am pretty sure it use to be the same for AMD but maybe it is now
enabled by default.

Cheers,
Jérôme