Re: [PATCH v3] vfio/type1: optimize vfio_pin_pages_remote() for huge folio

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, May 28, 2025 at 02:09:41PM -0600, Alex Williamson wrote:

> To be fair to libvirt, we'd really like libvirt to make use of iommufd
> whenever it's available, but without feature parity this would break
> users.  

If people ask there should be no issue with making API functionality
discoverable with a query command of some kind. Alot of new stuff is
already discoverable by invoking an ioctl with bogus arguments and
checking for EOPNOTSUPP/ENOTTY.

But most likely P2P will use the same ioctl as memfd so it will not
work that way.

So for now libvirt could assume no P2P support in iommufd. A simple
algorithm would be to look for more than 1 VFIO device. Or add an xml
"disable P2P" which is a useful thing anyhow.

> Zhe, so if you have no dependencies on P2P DMA within your device
> assignment VMs, the options above may be useful or at least a data
> point for comparison of type1 vs IOMMUFD performance.  Thanks,

Even if Zhe does have P2P DMA I have a feeling the OOT P2P patch may
be workable <shrug>

Jason