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From: Christian Brauner <[email protected]>
To: Alex Williamson <[email protected]>
Cc: [email protected], [email protected],
	[email protected],
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Subject: Re: [PATCH 0/2] eventfd: simplify signal helpers
Date: Fri, 14 Jul 2023 09:05:21 +0200	[thread overview]
Message-ID: <20230714-gauner-unsolidarisch-fc51f96c61e8@brauner> (raw)
In-Reply-To: <[email protected]>

On Thu, Jul 13, 2023 at 11:10:54AM -0600, Alex Williamson wrote:
> On Thu, 13 Jul 2023 12:05:36 +0200
> Christian Brauner <[email protected]> wrote:
> 
> > Hey everyone,
> > 
> > This simplifies the eventfd_signal() and eventfd_signal_mask() helpers
> > by removing the count argument which is effectively unused.
> 
> We have a patch under review which does in fact make use of the
> signaling value:
> 
> https://lore.kernel.org/all/[email protected]/

Huh, thanks for the link.

Quoting from
https://patchwork.kernel.org/project/kvm/patch/[email protected]/#25266856

> Reading an eventfd returns an 8-byte value, we generally only use it
> as a counter, but it's been discussed previously and IIRC, it's possible
> to use that value as a notification value.

So the goal is to pipe a specific value through eventfd? But it is
explicitly a counter. The whole thing is written around a counter and
each write and signal adds to the counter.

The consequences are pretty well described in the cover letter of
v6 https://lore.kernel.org/all/[email protected]/

> Since the eventfd counter is used as ACPI notification value
> placeholder, the eventfd signaling needs to be serialized in order to
> not end up with notification values being coalesced. Therefore ACPI
> notification values are buffered and signalized one by one, when the
> previous notification value has been consumed.

But isn't this a good indication that you really don't want an eventfd
but something that's explicitly designed to associate specific data with
a notification? Using eventfd in that manner requires serialization,
buffering, and enforces ordering.

I have no skin in the game aside from having to drop this conversion
which I'm fine to do if there are actually users for this btu really,
that looks a lot like abusing an api that really wasn't designed for
this.

       reply	other threads:[~2023-07-14  7:05 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <[email protected]>
2023-07-14  7:05 ` Christian Brauner [this message]
2023-07-14 15:24   ` [PATCH 0/2] eventfd: simplify signal helpers Jason Gunthorpe
2023-07-17  8:29   ` Grzegorz Jaszczyk
2023-07-17 19:08     ` Alex Williamson
2023-07-17 22:12       ` Jason Gunthorpe
2023-07-17 22:52         ` Alex Williamson
2023-07-18 15:56           ` Jason Gunthorpe
2023-07-13 10:05 Christian Brauner
2023-07-13 17:10 ` Alex Williamson

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