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

[Alex Williamson]

On Fri, 27 Feb 2026 09:07:48 -0800
David Matlack <dmatlack@xxxxxxxxxx> wrote:

> On Fri, Feb 27, 2026 at 7:47 AM Alex Williamson <alex@xxxxxxxxxxx> wrote:
> >
> > On Fri, 27 Feb 2026 00:51:18 +0000
> > David Matlack <dmatlack@xxxxxxxxxx> wrote:
> >  
> > > On 2026-02-26 05:00 PM, Alex Williamson wrote:  
> > > > On Thu, 29 Jan 2026 21:24:57 +0000
> > > > David Matlack <dmatlack@xxxxxxxxxx> wrote:  
> > > > >
> > > > > - vdev->reset_works = !ret;
> > > > >   pci_save_state(pdev);
> > > > >   vdev->pci_saved_state = pci_store_saved_state(pdev);  
> > > >
> > > > Isn't this a problem too?  In the first kernel we store the initial,
> > > > post reset state of the device, now we're storing some arbitrary state.
> > > > This is the state we're restore when the device is closed.  
> > >
> > > The previous kernel resets the device and restores it back to its
> > > post reset state in vfio_pci_liveupdate_freeze() before handing off
> > > control to the next kernel. So my intention here is that VFIO will
> > > receive the device in that state, allowing it to call
> > > pci_store_saved_state() here to capture the post reset state of the
> > > device again.
> > >
> > > Eventually we want to drop the reset in vfio_pci_liveupdate_freeze() and
> > > preserve vdev->pci_saved_state across the Live Update. But I was hoping
> > > to add that in a follow up series to avoid this one getting too long.  
> >
> > I appreciate reviewing this in smaller chunks, but how does userspace
> > know whether the kernel contains a stub implementation of liveupdate or
> > behaves according to the end goal?  
> 
> Would a new VFIO_DEVICE_INFO_CAP be a good way to communicate this
> information to userspace?

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?
Thanks,

Alex