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Date:   Tue, 18 Jul 2023 12:56:38 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Alex Williamson <alex.williamson@redhat.com>
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Subject: Re: [PATCH 0/2] eventfd: simplify signal helpers
Message-ID: <ZLa2NmwexoxPkS9a@ziepe.ca>
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On Mon, Jul 17, 2023 at 04:52:03PM -0600, Alex Williamson wrote:
> On Mon, 17 Jul 2023 19:12:16 -0300
> Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 
> > On Mon, Jul 17, 2023 at 01:08:31PM -0600, Alex Williamson wrote:
> > 
> > > What would that mechanism be?  We've been iterating on getting the
> > > serialization and buffering correct, but I don't know of another means
> > > that combines the notification with a value, so we'd likely end up with
> > > an eventfd only for notification and a separate ring buffer for
> > > notification values.  
> > 
> > All FDs do this. You just have to make a FD with custom
> > file_operations that does what this wants. The uAPI shouldn't be able
> > to tell if the FD is backing it with an eventfd or otherwise. Have the
> > kernel return the FD instead of accepting it. Follow the basic design
> > of eg mlx5vf_save_fops
> 
> Sure, userspace could poll on any fd and read a value from it, but at
> that point we're essentially duplicating a lot of what eventfd provides
> for a minor(?) semantic difference over how the counter value is
> interpreted.  Using an actual eventfd allows the ACPI notification to
> work as just another interrupt index within the existing vfio IRQ
> uAPI.

Yes, duplicated, sort of, whatever the "ack" is to allow pushing a new
value can be revised to run as part of the read.

But I don't really view it as a minor difference. eventfd is a
counter. It should not be abused otherwise, even if it can be made to
work.

It really isn't an IRQ if it is pushing an async message w/data.

Jason