On Fri, Feb 27, 2026 at 8:32 AM Alex Williamson <[email protected]> wrote: > > On Thu, 26 Feb 2026 00:28:28 +0000 > David Matlack <[email protected]> wrote: > > > > +static int pci_flb_preserve(struct liveupdate_flb_op_args *args) > > > > +{ > > > > + struct pci_dev *dev = NULL; > > > > + int max_nr_devices = 0; > > > > + struct pci_ser *ser; > > > > + unsigned long size; > > > > + > > > > + for_each_pci_dev(dev) > > > > + max_nr_devices++; > > > > > > How is this protected against hotplug? > > > > Pranjal raised this as well. Here was my reply: > > > > . Yes, it's possible to run out space to preserve devices if devices are > > . hot-plugged and then preserved. But I think it's better to defer > > . handling such a use-case exists (unless you see an obvious simple > > . solution). So far I am not seeing preserving hot-plugged devices > > . across Live Update as a high priority use-case to support. > > > > I am going to add a comment here in the next revision to clarify that. > > I will also add a comment clarifying why this code doesn't bother to > > account for VFs created after this call (preserving VFs are explicitly > > disallowed to be preserved in this patch since they require additional > > support). > > TBH, without SR-IOV support and some examples of in-kernel PF > preservation in support of vfio-pci VFs, it seems like this only > supports a very niche use case.
The intent is to start by supporting a simple use-case and expand to more complex scenarios over time, including preserving VFs. Full GPU passthrough is common at cloud providers so even non-VF preservation support is valuable. > I expect the majority of vfio-pci > devices are VFs and I don't think we want to present a solution where > the requirement is to move the PF driver to userspace. JasonG recommended the upstream support for VF preservation be limited to cases where the PF is also bound to VFIO: https://lore.kernel.org/lkml/[email protected]/ Within Google we have a way to support in-kernel PF drivers but we are trying to focus on simpler use-cases first upstream. > It's not clear, > for example, how we can have vfio-pci variant drivers relying on > in-kernel channels to PF drivers to support migration in this model. Agree this still needs to be fleshed out and designed. I think the roadmap will be something like: 1. Get non-VF preservation working end-to-end (device fully preserved and doing DMA continuously during Live Update). 2. Extend to support VF preservation where the PF is also bound to vfio-pci. 3. (Maybe) Extend to support in-kernel PF drivers. This series is the first step of #1. I have line of sight to how #2 could work since it's all VFIO.
