Re: [PATCH v3 6/7] docs: gpio: Add GPIO Aggregator/Repeater documentation

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Harish Jenny K N <harish_kandiga@mentor.com>,
	Eugeniu Rosca <erosca@de.adit-jv.com>,
	Alexander Graf <graf@amazon.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Phil Reid <preid@electromag.com.au>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Magnus Damm <magnus.damm@gmail.com>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [PATCH v3 6/7] docs: gpio: Add GPIO Aggregator/Repeater documentation
Date: Thu, 12 Dec 2019 15:48:38 +0100	[thread overview]
Message-ID: <CAMuHMdV+Ww5Y9G7+bS-SyHtm4dC89V37yuaYvrS3kYW=PRTkFA@mail.gmail.com> (raw)
In-Reply-To: <CACRpkdb1XZAeSThxWmJtnm80T4aPufXV2UvJdVdgnw-TJe3trg@mail.gmail.com>

Hi Linus,

On Thu, Dec 12, 2019 at 3:42 PM Linus Walleij <linus.walleij@linaro.org> wrote:
> On Wed, Nov 27, 2019 at 9:43 AM Geert Uytterhoeven
> <geert+renesas@glider.be> wrote:
> > +The GPIO Aggregator allows access control for individual GPIOs, by aggregating
> > +them into a new gpio_chip, which can be assigned to a group or user using
> > +standard UNIX file ownership and permissions.  Furthermore, this simplifies and
> > +hardens exporting GPIOs to a virtual machine, as the VM can just grab the full
> > +GPIO controller, and no longer needs to care about which GPIOs to grab and
> > +which not, reducing the attack surface.
> > +
> > +Aggregated GPIO controllers are instantiated and destroyed by writing to
> > +write-only attribute files in sysfs.
>
> I suppose virtual machines will have a lengthy config file where
> they specify which GPIO lines to pick and use for their GPIO
> aggregator, and that will all be fine, the VM starts and the aggregator
> is there and we can start executing.
>
> I would perhaps point out a weakness as with all sysfs and with the current
> gpio sysfs: if a process creates an aggregator device, and then that
> process crashes, what happens when you try to restart the process and
> run e.g. your VM again?
>
> Time for a hard reboot? Or should we add some design guidelines for
> these machines so that they can cleanly tear down aggregators
> previously created by the crashed VM?

No, the VM does not create the aggregator.

The idea is for the user to create one or more aggregators, set up
permissions on /dev/gpiochipX, and launch the VM, passing the aggregated
/dev/gpiochipX as parameters.
If the VM crashes, just launch it again.

Destroying the aggregators is a manual and independent process, after
the VM has exited.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds