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

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

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,

Alex 

> 
>  We discussed about that in the earlier version of the patch series.
>  Since we have different exit related handling, so to avoid confusion
>  we proceeded with 2 separate variants for the low power entry. Also,
>  we don't need any parameter for the first case.
> 
>  But, I can do the changes to make a single entry point, if we conclude
>  for that. 
> 
>  From my side, I have explored how the uAPI looks like if
>  we go with this approach.
> 
>  /*
>   * 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 re-entry to a low power state will be controlled through
>   * VFIO_DEVICE_LOW_POWER_REENTERY_DISABLE flag.
>   *
>   * If LOW_POWER_REENTERY_DISABLE flag is not set, the device may re-enter the
>   * low power state.  Access to mmap'd device regions is disabled on
>   * LOW_POWER_ENTRY and may only be resumed after calling LOW_POWER_EXIT.
>   *
>   * If LOW_POWER_REENTERY_DISABLE flag is set, then user needs to provide an
>   * eventfd for wake-up notification.  When the device moves out of the low
>   * power state for the wake-up, the host will not allow the device to re-enter
>   * a low power state without a subsequent user call to LOW_POWER_ENTRY.
>   * Access to mmap'd device regions is disabled on LOW_POWER_ENTRY and may only
>   * be resumed after the low power exit.  The low power exit can happen either
>   * through LOW_POWER_EXIT or through any other access (where the wake-up
>   * notification has been generated).  The access to mmap'd device regions will
>   * not trigger low power exit.
>   *
>   * The notification through the provided eventfd will be generated only when
>   * the device has entered and is resumed from a low power state after
>   * calling this device feature IOCTL.  A device that has not entered low power
>   * state, as managed through the runtime power management core, will not
>   * generate a notification through the provided eventfd on access.  Calling the
>   * LOW_POWER_EXIT feature is optional in the case where notification has been
>   * signaled on the provided eventfd that a resume from low power has occurred.
>   *
>   * The wakeup_eventfd needs to be valid only if LOW_POWER_REENTERY_DISABLE
>   * flag is set, otherwise, it will be ignored.
>   */
>  #define VFIO_DEVICE_FEATURE_LOW_POWER_ENTRY 3
>  
>  struct vfio_device_low_power_entry_with_wakeup {
>  	__u32 flags;
>  #define VFIO_DEVICE_LOW_POWER_REENTERY_DISABLE	(1 << 0)
>  	__s32 wakeup_eventfd;
>  };
>  
>  /*
>   * Upon VFIO_DEVICE_FEATURE_SET, disallow use of device low power states as
>   * previously enabled via VFIO_DEVICE_FEATURE_LOW_POWER_ENTRY device feature.
>   * This device feature IOCTL may itself generate a wakeup eventfd notification
>   * if the device had previously entered a low power state with
>   * VFIO_DEVICE_LOW_POWER_REENTERY_DISABLE flag set.
>   */
>  #define VFIO_DEVICE_FEATURE_LOW_POWER_EXIT 4
> 
>  Thanks,
>  Abhishek
>