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

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