Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Viresh,

On Fri, Jun 11, 2021 at 10:01 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 11-06-21, 09:42, Geert Uytterhoeven wrote:
> > On Fri, Jun 11, 2021 at 5:56 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > On 10-06-21, 22:46, Linus Walleij wrote:
> > > > thanks for working on this, it's a really interesting driver.
> > > >
> > > > My first question is conceptual:
> > > >
> > > > We previously have Geerts driver for virtualization:
> > > > drivers/gpio/gpio-aggregator.c
> > > >
> > > > The idea with the aggregator is that a host script sets up a
> > > > unique gpiochip for the virtualized instance using some poking
> > > > in sysfs and pass that to the virtual machine.
> > > > So this is Linux acting as virtualization host by definition.
> >
> > The gpio-aggregator is running on the host...
> >
> > > > I think virtio is more abstract and intended for the usecase
> > > > where the hypervisor is not Linux, so this should be mentioned
> > > > in the commit, possibly also in Kconfig so users immediately
> > > > know what usecases the two different drivers are for.
> >
> > ... while the virtio-gpio driver is meant for the guest kernel.
> >
> > I my PoC "[PATCH QEMU v2 0/5] Add a GPIO backend"[1], I didn't have
> > a virtio transport, but just hooked into the PL061 GPIO emulation
> > in QEMU.  The PL061 QEMU driver talked to the GPIO backend, which
> > talked to /dev/gpiochipN on the host.
>
> Hmm, interesting.
>
> > > Well, not actually.
> > >
> > > The host can actually be anything. It can be a Xen based dom0, which
> > > runs some proprietary firmware, or Qemu running over Linux.
> > >
> > > It is left for the host to decide how it wants to club together the
> > > GPIO pins from host and access them, with Linux host userspace it
> > > would be playing with /dev/gpiochipN, while for a raw one it may
> > > be accessing registers directly.
> > >
> > > And so the backend running at host, needs to pass the gpiochip
> > > configurations and only the host understand it.
> >
> > So QEMU has to translate the virtio-gpio communication to e.g.
> > /dev/gpiochipN on the host (or a different backend on non-Linux or
> > bare-metal HV).
>
> No, QEMU passes the raw messages to the backend daemon running in host
> userspace (which shares a socket with qemu). The backend understands
> the virtio/vhost protocols and so won't be required to change at all
> if we move from Qemu to something else. And that's what we (Linaro)
> are looking to do here with Project Stratos.
>
> Create virtio based hypervisor agnostic backends.

OK, IC.

> > > The way I test it for now is by running this with Qemu over my x86
> > > box, so my host side is indeed playing with sysfs Linux.
> >
> > Can you please share a link to the QEMU patches?
>
> Unfortunately, they aren't in good shape right now and the backend is
> a bit hacky (Just checking the data paths, but not touching
> /dev/gpiochipN at all for now).
>
> I didn't implement one as I am going to implement the backend in Rust
> and not Qemu. So it doesn't depend on Qemu at all.

OK.

> To give you an idea of the whole thing, here is what we have done for
> I2c for example, GPIO one will look very similar.

Oh, you also have i2c. Nice!

> The Qemu patches:
>
> https://yhbt.net/lore/all/cover.1617278395.git.viresh.kumar@linaro.org/T/
>
> The stuff from tools/vhost-user-i2c/ directory (or patch 4/6) isn't
> used anymore and the following Rust implementation replaces it:
>
> https://github.com/vireshk/vhost-device/tree/master/src/i2c

Thanks for the links!

> > The GPIO aggregator came into play after talking to Alexander Graf and
> > Peter Maydell.  To reduce the attack surface, they didn't want QEMU
> > to be responsible for exporting to the guest a subset of all GPIOs of
> > a gpiochip, only a full gpiochip.  However, the full gpiochip may
> > contain critical GPIOs you do not want the guest to tamper with.
> > Hence the GPIO aggregator was born, to take care of aggregating all
> > GPIOs you want to export to a guest into a new virtual gpiochip.
> >
> > You can find more information about the GPIO Aggregator's use cases in
> > "[PATCH v7 0/6] gpio: Add GPIO Aggregator"[2].
>
> So I was actually looking to do some kind of aggregation on the host
> side's backend daemon to share only a subset of GPIO pins, I will see
> if that is something I can reuse. Thanks for sharing details.

The same reasoning can apply to your backend daemon, so when using
the GPIO aggregator, you can just control a full gpiochip, without
having to implement access control on individual GPIO lines.

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