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

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Message-ID: <YulSOK+YKjV2634b@nvidia.com>
Date:   Tue, 2 Aug 2022 13:35:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Abhishek Sahu <abhsahu@...dia.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, Aug 02, 2022 at 09:41:28AM -0600, Alex Williamson wrote:

> The subtlety is that there's a flag and a field and the flag can only
> be set if the field is set, the flag can only be clear if the field is
> clear, so we return -EINVAL for the other cases?  Why do we have both a
> flag and a field?  This isn't like we're adding a feature later and the
> flag needs to indicate that the field is present and valid.  It's just
> not a very clean interface, imo.  Thanks,

That isn't how I read Abhishek's proposal.. The eventfd should always
work and should always behave as described "The notification through
the provided eventfd will be generated only when the device has
entered and is resumed from a low power state"

If userspace provides it without LOW_POWER_REENTERY_DISABLE then it
still generates the events.

The linkage to LOW_POWER_REENTERY_DISABLE is only that userspace
probably needs to use both elements together to generate the
auto-reentry behavior. Kernel should not enforce it.

Two fields, orthogonal behaviors.

Jason