Re: [RFC 00/18] vfio: Adopt iommufd

Alex Williamson
2022-04-26T19:24:44Z

On Tue, 26 Apr 2022 13:42:17 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Apr 26, 2022 at 10:21:59AM -0600, Alex Williamson wrote:
> > We also need to be able to advise libvirt as to how each iommufd object
> > or user of that object factors into the VM locked memory requirement.
> > When used by vfio-pci, we're only mapping VM RAM, so we'd ask libvirt
> > to set the locked memory limit to the size of VM RAM per iommufd,
> > regardless of the number of devices using a given iommufd. However, I
> > don't know if all users of iommufd will be exclusively mapping VM RAM.
> > Combinations of devices where some map VM RAM and others map QEMU
> > buffer space could still require some incremental increase per device
> > (I'm not sure if vfio-nvme is such a device). It seems like heuristics
> > will still be involved even after iommufd solves the per-device
> > vfio-pci locked memory limit issue. Thanks,
>
> If the model is to pass the FD, how about we put a limit on the FD
> itself instead of abusing the locked memory limit?
>
> We could have a no-way-out ioctl that directly limits the # of PFNs
> covered by iopt_pages inside an iommufd.

FD passing would likely only be the standard for libvirt invoked VMs.
The QEMU vfio-pci device would still parse a host= or sysfsdev= option
when invoked by mortals and associate to use the legacy vfio group
interface or the new vfio device interface based on whether an iommufd
is specified.

Does that rule out your suggestion? I don't know, please reveal more
about the mechanics of putting a limit on the FD itself and this
no-way-out ioctl. The latter name suggests to me that I should also
note that we need to support memory hotplug with these devices. Thanks,

Alex