Re: [PATCH v2 10/22] vfio/pci: Skip reset of preserved device after Live Update

David Matlack
2026-03-16T17:18:57Z

On Mon, Mar 16, 2026 at 9:22 AM Vipin Sharma <vipinsh@google.com> 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 <alex@shazbot.org> 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.