Re: [PATCH v4 02/16] vfio/pci: Preserve vfio-pci device files across Live Update

[Vipin Sharma <vipinsh@google.com>]

On Tue, May 12, 2026 at 01:59:51PM -0700, David Matlack wrote:
> On Mon, May 11, 2026 at 4:48 PM Vipin Sharma <vipinsh@google.com> wrote:
> 
> > diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> > index c12d614fc6c4..019de053f116 100644
> > --- a/drivers/vfio/pci/Kconfig
> > +++ b/drivers/vfio/pci/Kconfig
> > @@ -45,13 +45,15 @@ config VFIO_PCI_IGD
> >
> >  config VFIO_PCI_LIVEUPDATE
> >         bool "VFIO PCI support for Live Update (EXPERIMENTAL)"
> > -       depends on PCI_LIVEUPDATE
> > +       depends on PCI_LIVEUPDATE && VFIO_DEVICE_CDEV
> >         help
> >           Support for preserving devices bound to vfio-pci across a Live
> >           Update. 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.
> >
> > +         Enabling this will disable support for VFIO PCI DMA buffer.
> > +
> >           If you don't know what to do here, say N.
> >
> >  endif
> > @@ -68,7 +70,7 @@ config VFIO_PCI_ZDEV_KVM
> >           To enable s390x KVM vfio-pci extensions, say Y.
> >
> >  config VFIO_PCI_DMABUF
> > -       def_bool y if VFIO_PCI_CORE && PCI_P2PDMA && DMA_SHARED_BUFFER
> > +       def_bool y if VFIO_PCI_CORE && PCI_P2PDMA && DMA_SHARED_BUFFER && !VFIO_PCI_LIVEUPDATE
> 
> Why does enabling VFIO_PCI_LIVEUPDATE require disabling
> VFIO_PCI_DMABUF? I saw the cover letter says "to keep things simple",
> but what specific problem does this solve or simplify?

I should have provided more details there.

When device is getting reset in vfio_pci_liveupdate_freeze(), we are
zapping userspace mapped bars, we also need to use
vfio_pci_dma_buf_move() to revoke dma buffer access or
vfio_pci_dma_buf_cleanup() combination. Cleanup takes the memory lock
which freeze already takes, and there are some refcounts which are
managed in both of these APIs. This was causing complexities with code
flow based on result of pci_load_saved_state(). All this was adding more
refactoring than I wanted in the series.

I decided to just drop the change and disable the support of DMA Buffer for
now to keep number of patches less in the series. This will go away once
we remove reset condition in freeze.