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? I am not sure providing VFIO_PCI_LIVEUPDATE alleviate Alex's concern about how userspace will know that sufficient VFIO support exists. May be write in liveupdate documentation (PATCH 11 of this series) that support is experimental?
