Re: [PATCH V2 3/7] vhost-vdpa: VHOST_NEW_OWNER

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 15, 2024 at 10:29:26AM -0400, Steven Sistare wrote:
> On 7/15/2024 5:07 AM, Michael S. Tsirkin wrote:
> > On Fri, Jul 12, 2024 at 06:18:49AM -0700, Steve Sistare wrote:
> > > Add an ioctl to transfer file descriptor ownership and pinned memory
> > > accounting from one process to another.
> > > 
> > > This is more efficient than VHOST_RESET_OWNER followed by VHOST_SET_OWNER,
> > > as that would unpin all physical pages, requiring them to be repinned in
> > > the new process.  That would cost multiple seconds for large memories, and
> > > be incurred during a virtual machine's pause time during live update.
> > > 
> > > Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> > 
> > Please, we just need to switch to use iommufd for pinning.
> > Piling up all these hacks gets us nowhere.
> 
> I am working on iommufd kernel interfaces and QEMU changes.  But who is working
> on iommufd support for vdpa? If no one, or not for years, then adding these
> small interfaces to vdpa plugs a signficant gap in live update coverage.
> 
> FWIW, the iommufd interfaces for live update will look much the same: change owner
> and pinned memory accounting, and update virtual addresses.  So adding that to vdpa
> will not make it look like an odd duck.
> 
> - Steve

I think that no one is working on it - Cindy posted some rfcs in January
("vhost-vdpa: add support for iommufd").  Feel free to pick that up.
What you described is just more of a reason not to duplicate this code.
And it's always the same: a small extension here, a small extension there.
If you can make do with existing kernel interfaces, fine,
one can argue that userspace code is useful to support existing kernels.

-- 
MST