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

From: Abhishek Sahu <abhsahu@nvidia.com>
To: Jason Gunthorpe <jgg@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>
Cc: 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: Wed, 3 Aug 2022 12:02:06 +0530	[thread overview]
Message-ID: <d90a6a11-f6c4-9e29-61a1-c96f1ceb100b@nvidia.com> (raw)
In-Reply-To: <YulYVOWh8km2knhx@nvidia.com>

On 8/2/2022 10:31 PM, Jason Gunthorpe wrote:
> 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

 Thanks Alex and Jason for your inputs.

 It seems, I can use the original uAPI where we will have 2 variants
 of ENTRY.

 Since already kernel merge window is started, so I will wait for
 v5.20-rc1 (or v6.0-rc1) and then I will rebase and test my patches on
 the updated kernel.

 Regards,
 Abhishek