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

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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