[Intel-gfx] [PATCH v2 10/11] vfio: Make vfio_container optionally compiled

[Alex Williamson]

On Thu, 10 Nov 2022 06:57:57 +0000
"Tian, Kevin" <kevin.tian at intel.com> wrote:

> > From: Jason Gunthorpe <jgg at nvidia.com>
> > Sent: Thursday, November 10, 2022 3:53 AM
> > 
> > On Wed, Nov 09, 2022 at 10:18:09AM -0700, Alex Williamson wrote:
> >   
> > > DPDK supports no-iommu mode.  
> > 
> > Er? Huh? How? I thought no-iommu was for applications that didn't do
> > DMA? How is DPDK getting packets in/out without DMA? I guess it snoops
> > in /proc/ or something to learn PFNs of mlock'd memory? <shudder>  
> 
> iirc dpdk started with UIO plus various tricks (root privilege, hugepage, etc.)
> to lock and learn PFN's from pagemap. Then when migrating it to vfio the
> no-iommu option was introduced to provide UIO compatibility.

IIRC, we essentially introduced no-iommu mode vfio because DPDK started
pushing for extending interrupt support in uio-pci-generic.  The UIO
driver is also only meant for devices that don't do DMA, but obviously
DPDK didn't care about that.  Rather than extend UIO, we offered this
no-iommu mode in vfio since we already had more extensive MSI support,
were better able to impose restrictions on access to the device, and
using the same device access makes the transition to proper IOMMU
backed configurations more seamless.  Thanks,

Alex