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

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Date:   Tue, 26 Jul 2022 14:23:56 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Abhishek Sahu <abhsahu@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Kevin Tian <kevin.tian@...el.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v5 1/5] vfio: Add the device features for the low power
 entry and exit

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