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

From: Jason Gunthorpe <jgg@nvidia.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Abhishek Sahu <abhsahu@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Yishai Hadas <yishaih@nvidia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
	Kevin Tian <kevin.tian@intel.com>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Max Gurtovoy <mgurtovoy@nvidia.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	linux-pm@vger.kernel.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH v5 1/5] vfio: Add the device features for the low power entry and exit
Date: Tue, 2 Aug 2022 14:01:08 -0300	[thread overview]
Message-ID: <YulYVOWh8km2knhx@nvidia.com> (raw)
In-Reply-To: <20220802105755.2ee80696.alex.williamson@redhat.com>

On Tue, Aug 02, 2022 at 10:57:55AM -0600, Alex Williamson wrote:
> On Tue, 2 Aug 2022 13:35:04 -0300
> Jason Gunthorpe <jgg@nvidia.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? 

I don't know - the eventfds is counting so it does let userspace
monitor frequency of auto-sleeping.

In any case the point is to make simple kernel APIs, not cover every
combination with a use case. Decoupling is simpler than coupling.

Jason