Re: [PATCH] make vfio and DAX cache work together

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 26 Apr 2021 21:50:38 +0100
Dev Audsin <dev.devaqemu@gmail.com> wrote:

> Hi Alex and David
> 
> @Alex:
> 
> Justification on why this region cannot be a DMA target for the device,
> 
> virtio-fs with DAX is currently not compatible with NIC Pass through.
> When a SR-IOV VF attaches to a qemu process, vfio will try to pin the
> entire DAX Window but it is empty when the guest boots and will fail.
> A method to make VFIO and DAX to work together is to make vfio skip
> DAX cache.
> 
> Currently DAX cache need to be set to 0, for the SR-IOV VF to be
> attached to Kata containers. Enabling both SR-IOV VF and DAX work
> together will potentially improve performance for workloads which are
> I/O and network intensive.

Sorry, there's no actual justification described here.  You're enabling
a VM with both features, virtio-fs DAX and VFIO, but there's no
evidence that they "work together" or that your use case is simply
avoiding a scenario where the device might attempt to DMA into the area
with this designation.  With this change, if the device were to attempt
to DMA into this region, it would be blocked by the IOMMU, which might
result in a data loss within the VM.  Justification of this change
needs to prove that this region can never be a DMA target for the
device, not simply that both features can be enabled and we hope that
they don't interact.  Thanks,

Alex