On Mon, Mar 16, 2026 at 9:22 AM Vipin Sharma <[email protected]> wrote: > > On Thu, Mar 12, 2026 at 11:39:45PM +0000, David Matlack wrote: > > On 2026-03-09 10:32 AM, David Matlack wrote: > > > On Fri, Feb 27, 2026 at 9:57 AM Alex Williamson <[email protected]> wrote: > > > > > > Sorry if I don't have the whole model in my head yet, but is exposing > > > > the restriction to the vfio user of the device sufficient to manage the > > > > liveupdate orchestration? For example, a VFIO_DEVICE_INFO_CAP pushes > > > > the knowledge to QEMU... what does QEMU do with that knowledge? Who > > > > imposes the policy decision to decide what support is sufficient? > > > > > > Hm.. good questions. I don't think we want userspace inspecting bits > > > exposed by the kernel and trying to infer exactly what's being > > > preserved and whether it's "good enough" to use. And such a UAPI would > > > become tech debt once we finish development, I suspect. > > > > > > A better approach would be to hide this support from userspace until > > > we decide it is ready for production use-cases. > > > > > > To enable development and testing, we can add an opt-in mechanism > > > > Here is what I am trending towards sending in v3 as the opt-in mechanism: > > > > diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig > > index 1e82b44bda1a..770231554221 100644 > > --- a/drivers/vfio/pci/Kconfig > > +++ b/drivers/vfio/pci/Kconfig > > @@ -58,6 +58,27 @@ config VFIO_PCI_ZDEV_KVM > > config VFIO_PCI_DMABUF > > def_bool y if VFIO_PCI_CORE && PCI_P2PDMA && DMA_SHARED_BUFFER > > > > +config VFIO_PCI_LIVEUPDATE > > + bool "VFIO PCI support for Live Update (EXPERIMENTAL)" > > + depends on LIVEUPDATE && VFIO_PCI > > + help > > + Support for preserving devices bound to vfio-pci across a Live > > + Update. The eventual goal is that preserved devices can run > > + uninterrupted during a Live Update, including DMA to preserved > > + memory buffers and P2P. However there are many steps still needed > > to > > + achieve this, including: > > + > > + - Preservation of iommufd files > > + - Preservation of IOMMU driver state > > + - Preservation of PCI state (BAR resources, device state, ...) > > + - Preservation of vfio-pci driver state > > + > > + This option should only be enabled by developers working on > > + implementing this support. Once enough support has landed in the > > + kernel, this option will no longer be marked EXPERIMENTAL. > > + > > + If you don't know what to do here, say N. > > + > > To use VFIO liveupdate, user has to do at least two things: > 1. Enable CONFIG_LIVEUPDATE > 2. Pass VFIO FD to a live update session. > > This means someone using it has to know what live update is and > intentionally pass the VFIO FDs. Isn't act of doing this itself an > opt-in mechanism?
If it is, then I can leave this out. Alex? My thinking was: Distros are free to enable LIVEUPDATE and use it. The support it enables today is all fully functional (albeit new). vfio-cdev, OTOH, is not. A separate Kconfig can help express that difference. Consider that LIVEUPDATE could be enabled by default in a future release, but vfio-cdev support might not be ready yet at that point. > I am not sure providing VFIO_PCI_LIVEUPDATE alleviate Alex's concern > about how userspace will know that sufficient VFIO support exists. I was thinking we can flip VFIO_PCI_LIVEUPDATE to be enabled by default (if LIVEUPDATE and VFIO_PCI are enabled), and drop "(EXPERIMENTAL)" from the option title. That would be how distros and downstream users of the kernel know that sufficient support exists to enable VFIO_PCI_LIVEUPDATE. > May be write in liveupdate documentation (PATCH 11 of this series) that > support is experimental? The documentation in patch 11 includes largely the same text that I put under VFIO_PCI_LIVEU"PDATE. But I can explicitly mention "experimental" as well if that's what you're asking.
