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

[Jason Gunthorpe <jgg@nvidia.com>]

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..

> 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?

Jason