Re: [PATCH v3 8/8] vfio/pci: Add the support for PCI D3cold state

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 1 Jun 2022 14:30:54 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Wed, Jun 01, 2022 at 10:21:51AM -0600, Alex Williamson wrote:
> 
> > Some ioctls clearly cannot occur while the device is in low power, such
> > as resets and interrupt control, but even less obvious things like
> > getting region info require device access.  Migration also provides a
> > channel to device access.    
> 
> I wonder what power management means in a case like that.
> 
> For the migration drivers they all rely on a PF driver that is not
> VFIO, so it should be impossible for power management to cause the PF
> to stop working.
> 
> I would expect any sane design of power management for a VF to not
> cause any harm to the migration driver..

Is there even a significant benefit or use case for power management
for VFs?  The existing D3hot support should be ok, but I imagine to
support D3cold, all the VFs and the PF would need to move to low power.
It might be safe to simply exclude VFs from providing this feature for
now.

> > I'm also still curious how we're going to handle devices that cannot
> > return to low power such as the self-refresh mode on the GPU.  We can
> > potentially prevent any wake-ups from the vfio device interface, but
> > that doesn't preclude a wake-up via an external lspci.  I think we need
> > to understand how we're going to handle such devices before we can
> > really complete the design.  AIUI, we cannot disable the self-refresh
> > sleep mode without imposing unreasonable latency and memory
> > requirements on the guest and we cannot retrigger the self-refresh
> > low-power mode without non-trivial device specific code.  
> 
> It begs the question if power management should be something that only
> a device-specific drivers should allow?

Yes, but that's also penalizing devices that require no special
support, for the few that do.  I'm not opposed to some sort of
vfio-pci-nvidia-gpu variant driver to provide that device specific
support, but I'd think the device table for such a driver might just be
added to the exclusion list for power management support in vfio-pci.
vfio-pci-core would need some way for drivers to opt-out/in for power
management.  Thanks,

Alex