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

[Vipin Sharma <vipinsh@google.com>]

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?

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?