Re: vfio/pci - uAPI for WC

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Alex, Cornelia, kvm, lkml (from "get_maintainer.pl drivers/vfio")
and rewrapped for plain-text readability]
On Thu, Aug 04, 2022 at 09:47:36AM +0000, Arinzon, David wrote:
> Hi,
> 
> There's currently no mechanism for vfio that exposes WC-related
> operations (check if memory is WC capable, ask to WC memory) to user
> space module entities, such as DPDK, for example.
>
> This topic has been previously discussed in [1], [2] and [3], but
> there was no follow-up.
>
> This capability is very useful for DPDK, specifically to the DPDK
> ENA driver that uses vfio-pci, which requires memory to be WC on the
> TX path. Without WC, higher CPU utilization and performance
> degradation are observed.
>
> In the above mentioned discussions, three options were suggested:
> sysfs, ioctl, mmap extension (extra attributes).
> 
> Was there any progress on this area? Is there someone who's looking
> into this?
>
> We're leaning towards the ioctl option, if there are no objections,
> we'd come up with an RFC.

> [1]: https://patchwork.kernel.org/project/kvm/patch/20171009025000.39435-1-aik@ozlabs.ru/
> [2]: https://lore.kernel.org/linux-pci/2b539df4c9ec703458e46da2fc879ee3b310b31c.camel@kernel.crashing.org/
> [3]: https://lore.kernel.org/lkml/20210429162906.32742-2-sdonthineni@nvidia.com/
> 
> Thanks,
> David