Re: [PATCH v5 1/5] vfio: Add the device features for the low power entry and exit

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Aug 01, 2022 at 12:42:53PM -0600, Alex Williamson wrote:
> On Wed, 27 Jul 2022 11:37:02 +0530
> Abhishek Sahu <abhsahu@nvidia.com> wrote:
> 
> > On 7/26/2022 10:53 PM, Jason Gunthorpe wrote:
> > > On Tue, Jul 26, 2022 at 06:17:18PM +0530, Abhishek Sahu wrote:  
> > >>  Thanks Alex for your thorough review of uAPI.
> > >>  I have incorporated all the suggestions.
> > >>  Following is the updated uAPI.
> > >>  
> > >>  /*
> > >>   * Upon VFIO_DEVICE_FEATURE_SET, allow the device to be moved into a low power
> > >>   * state with the platform-based power management.  Device use of lower power
> > >>   * states depends on factors managed by the runtime power management core,
> > >>   * including system level support and coordinating support among dependent
> > >>   * devices.  Enabling device low power entry does not guarantee lower power
> > >>   * usage by the device, nor is a mechanism provided through this feature to
> > >>   * know the current power state of the device.  If any device access happens
> > >>   * (either from the host or through the vfio uAPI) when the device is in the
> > >>   * low power state, then the host will move the device out of the low power
> > >>   * state as necessary prior to the access.  Once the access is completed, the
> > >>   * device may re-enter the low power state.  For single shot low power support
> > >>   * with wake-up notification, see
> > >>   * VFIO_DEVICE_FEATURE_LOW_POWER_ENTRY_WITH_WAKEUP below.  Access to mmap'd
> > >>   * device regions is disabled on LOW_POWER_ENTRY and may only be resumed after
> > >>   * calling LOW_POWER_EXIT.
> > >>   */
> > >>  #define VFIO_DEVICE_FEATURE_LOW_POWER_ENTRY 3
> > >>  
> > >>  /*
> > >>   * This device feature has the same behavior as
> > >>   * VFIO_DEVICE_FEATURE_LOW_POWER_ENTRY with the exception that the user
> > >>   * provides an eventfd for wake-up notification.  
> > > 
> > > It feels like this should be one entry point instead of two.
> > > 
> > > A flag "automatic re-sleep" and an optional eventfd (-1 means not
> > > provided) seems to capture both of these behaviors in a bit clearer
> > > and extendable way.
> 
> I think the mutual exclusion between re-entrant mode and one-shot is
> quite a bit more subtle in the version below, so I don't particularly
> find this cleaner.  Potentially we could have variant drivers support
> one w/o the other in the previously proposed model as well.  It's
> interesting to see this suggestion since since we seem to have a theme
> of making features single purpose elsewhere.  Thanks,

It is still quite single purpose, just
VFIO_DEVICE_LOW_POWER_REENTERY_DISABLE is some minor customization of
that single purpose.

Either the flag is set or not, it isn't subtle..

Jason