Re: [Qemu-devel] [PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver

[Marc Zyngier]

On 01/08/2019 09:41, Linus Walleij wrote:
> Hi Geert!
> 
> Thanks for this very interesting patch!
> 
> On Fri, Jul 5, 2019 at 6:05 PM Geert Uytterhoeven
> <address@hidden> wrote:
> 
>> GPIO controllers are exported to userspace using /dev/gpiochip*
>> character devices.  Access control to these devices is provided by
>> standard UNIX file system permissions, on an all-or-nothing basis:
>> either a GPIO controller is accessible for a user, or it is not.
>> Currently no mechanism exists to control access to individual GPIOs.
> 
> Yes, I did that decision deliberately, as the chip is one device
> and the base system control is usually on a per-device granularity.
> At one point some people were asking for individual GPIO line
> permissions in the character device and my argument was something
> like why can't I have individual control over the access rights on a block
> device or the pixels on a graphics device then.
> 
> Jokes aside, filesystems do provide access control over individual
> blocks on a block device in a way. So it is further up the stack.
> 
> The same goes for this: something above the GPIO chip provide
> more granular access control, and as such it fits the need very well.
> 
>> Hence add a virtual GPIO driver to aggregate existing GPIOs (up to 32),
>> and expose them as a new gpiochip.  This is useful for implementing
>> access control, and assigning a set of GPIOs to a specific user.
>> Furthermore, it would simplify and harden exporting GPIOs to a virtual
>> machine, as the VM can just grab the full virtual GPIO controller, and
>> no longer needs to care about which GPIOs to grab and which not,
>> reducing the attack surface.
> 
> Excellent approach.
> 
> I would even go so far as to call it "gpio-virtualization" or
> "gpio-virtualized" rather than "gpio-virtual" so it is clear what the
> intended usecase is. We have a bit of confusion in the kernel
> because people misuse the word "virtual" left and right, like for
> "virtual IRQ number" (Linux IRQ numbers) which are just some
> random number space, but not really "virtual", it's a semantic
> disease similar to the confusion of using the word "manual" in
> computer code.

I'd drop the notion of "virtual" altogether. Nothing is virtual in this
thing at all (the GPIOs are very real, from what I gather). Instead (and
assuming I got it right, which is a long shot), what you have is a
"synthetic" GPIO controller, made from the GPIOs belonging to other
controllers. I'd call it "GPIO aggregator".

> 
> Here it is however used correctly! (Maybe for the first time.)
> 
>> Virtual GPIO controllers are instantiated by writing to the "new_device"
>> attribute file in sysfs:
>>
>>     $ echo "<gpiochipA> <gpioA1> [<gpioA2> ...]"
>>            "[, <gpiochipB> <gpioB1> [<gpioB2> ...]] ...]"
>>             > /sys/bus/platform/drivers/gpio-virt-agg/new_device
>>
>> Likewise, virtual GPIO controllers can be destroyed after use:
>>
>>     $ echo gpio-virt-agg.<N> \
>>             > /sys/bus/platform/drivers/gpio-virt-agg/delete_device
> 
> I suppose this is the right way to use sysfs.
> 
> I would check with some virtualization people (paged Marc Zyngier
> and Christoffer Dall) so they can say whether this is the way any
> virtual machine wants to populate its local GPIO chip for
> use with a certain machine.
> 
> If QEMU can deal in a simple and straight-forward way with this
> I see it quickly becoming a very useful tool for industrial automation
> where you want to run each control system in isolation and just
> respawn the virtual machine if something goes wrong.

What the VMM (QEMU, kvmtool) would need to do is to present this as a
"standard" GPIO IP, and use the backend aggregator to emulate it.
Certainly doable. The nice part is that all the work is in userspace,
and thus completely off my plate! ;-)

You also may want to look into not emulating a standard IP, but use
something virtio-based instead. The ACRN project seems to have something
like this in progress, but I know nothing about it.

Thanks,

        M.
-- 
Jazz is not dead. It just smells funny...