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, 2 Aug 2022 10:57:55 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.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, 2 Aug 2022 13:35:04 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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.

What's the point of notifying userspace that the device was resumed if
it might already be suspended again by the time userspace responds to
the eventfd?  This really muddies the single-shot vs re-entrant
behavior.  If userspace allows the kernel to re-enter low power, then
they don't need to be notified on every wake.  If userspace wants to be
notified on wake, userspace can also re-initiate low power entry.
Thanks,

Alex