Message-ID: <20230714-gauner-unsolidarisch-fc51f96c61e8@brauner>
Date: Fri, 14 Jul 2023 09:05:21 +0200
From: Christian Brauner <brauner@...nel.org>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	linux-usb@...r.kernel.org, Matthew Rosato <mjrosato@...ux.ibm.com>,
	Paul Durrant <paul@....org>, Tom Rix <trix@...hat.com>,
	Jason Wang <jasowang@...hat.com>, dri-devel@...ts.freedesktop.org,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	Kirti Wankhede <kwankhede@...dia.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Vineeth Vijayan <vneethv@...ux.ibm.com>,
	Diana Craciun <diana.craciun@....nxp.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Shakeel Butt <shakeelb@...gle.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Leon Romanovsky <leon@...nel.org>,
	Harald Freudenberger <freude@...ux.ibm.com>,
	Fei Li <fei1.li@...el.com>, x86@...nel.org,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Halil Pasic <pasic@...ux.ibm.com>, Jason Gunthorpe <jgg@...pe.ca>,
	Ingo Molnar <mingo@...hat.com>, intel-gfx@...ts.freedesktop.org,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	linux-fpga@...r.kernel.org, Zhi Wang <zhi.a.wang@...el.com>,
	Wu Hao <hao.wu@...el.com>, Jason Herne <jjherne@...ux.ibm.com>,
	Eric Farman <farman@...ux.ibm.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andrew Donnellan <ajd@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
	linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>, linuxppc-dev@...ts.ozlabs.org,
	Eric Auger <eric.auger@...hat.com>, Borislav Petkov <bp@...en8.de>,
	kvm@...r.kernel.org, Rodrigo Vivi <rodrigo.vivi@...el.com>,
	cgroups@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	virtualization@...ts.linux-foundation.org,
	intel-gvt-dev@...ts.freedesktop.org, io-uring@...r.kernel.org,
	netdev@...r.kernel.org, Tony Krowiak <akrowiak@...ux.ibm.com>,
	Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
	Pavel Begunkov <asml.silence@...il.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Oded Gabbay <ogabbay@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Peter Oberparleiter <oberpar@...ux.ibm.com>,
	linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Frederic Barrat <fbarrat@...ux.ibm.com>,
	Moritz Fischer <mdf@...nel.org>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Xu Yilun <yilun.xu@...el.com>, jaz@...ihalf.com
Subject: Re: [PATCH 0/2] eventfd: simplify signal helpers

On Thu, Jul 13, 2023 at 11:10:54AM -0600, Alex Williamson wrote:
> On Thu, 13 Jul 2023 12:05:36 +0200
> Christian Brauner <brauner@...nel.org> 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/20230630155936.3015595-1-jaz@semihalf.com/

Huh, thanks for the link.

Quoting from
https://patchwork.kernel.org/project/kvm/patch/20230307220553.631069-1-jaz@semihalf.com/#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/20230630155936.3015595-1-jaz@semihalf.com/

> 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.

Powered by blists - more mailing lists