Re: [PATCH v4 08/11] PCI: liveupdate: Require preserved devices are in immutable singleton IOMMU groups

[David Matlack <dmatlack@google.com>]

On 2026-04-23 11:27 PM, Samiullah Khawaja wrote:
> On Thu, Apr 23, 2026 at 04:09:01PM -0700, David Matlack wrote:
> > On Thu, Apr 23, 2026 at 3:53 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> > > 
> > > On Thu, Apr 23, 2026 at 03:10:55PM -0700, David Matlack wrote:
> > > > On Thu, Apr 23, 2026 at 2:23 PM David Matlack <dmatlack@google.com> wrote:
> > > > >
> > > > > Restrict support for preserving PCI devices across Live Update to
> > > > > devices in immutable singleton IOMMU groups. A device's group is
> > > > > considered immutable if all bridges upstream from the device up to the
> > > > > root port have the required ACS features enabled.
> > > > >
> > > > > Since ACS flags are inherited across a Live Update for preserved devices
> > > > > and all the way up to the root port, the preserved device should be in a
> > > > > singleton IOMMU group after kexec in the new kernel.
> > > > >
> > > > > This change should still permit all the current use-cases for PCI device
> > > > > preservation across Live Update, since it is intended to be used in
> > > > > Cloud enviroments which should have the required ACS features enabled
> > > > > for virtualization purposes.
> > > > >
> > > > > If a device is part of a multi-device IOMMU group, preserving it will
> > > > > now fail with an error. This restriction may be lifted in the future if
> > > > > support for preserving multi-device groups is desired.
> > > > >
> > > > > Signed-off-by: David Matlack <dmatlack@google.com>
> > > >
> > > > Jason, do you think requiring singleton iommu groups is still
> > > > necessary/useful now that this series preserves ACS flags on preserved
> > > > devices and upstream bridges?
> > > 
> > > I have forgotten why we introduced that? There are alot of funky
> > > things about iommu groups that might be important upon restoration..
> > 
> > You had originally suggested it in this thread:
> > 
> >  https://lore.kernel.org/kvm/20260301192236.GQ5933@nvidia.com/
> > 
> > > Like if you preserve one group member but not the other what do you ?
> > 
> > Yeah I imagine there could be some tricky cases there...
> > 
> > I wonder if PCI core is the right layer to enforce this. Maybe this
> > fits better into Sami's IOMMU core series since that is where all
> > those tricky cases will be (I imagine?).
> 
> +1
> 
> Also I think this should probably be checked by iommufd and invoked
> through vfio cdev. Basically when vfio cdev calls into iommufd to
> preserve IOMMU specific aspects of device (PASID table etc), iommufd can
> check this and return error.

Ok I will drop this patch from v5. The IOMMU core can check for it if it
makes life simpler, but I can't think of anything in the PCI core that
cares about this check.