Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

On Mon, Jun 28, 2021 at 11:25 AM Wolfram Sang [off-list ref] wrote:

quoted

As far as I understand me (please clarify), implementing only the smbus
subset would mean that we cannot communicate with all client devices,
while implementing both would add more complexity than the lower-level
protocol.

Correct. You need to pick I2C if you want to support all devices. SMBus
will give you only a lot of devices.

Ok, that's what I thought. There is one corner case that I've struggled
with though: Suppose the host has an SMBus-only driver, and the
proposed driver exposes this as an I2C device to the guest, which
makes it available to guest user space (or a guest kernel driver)
using the emulated smbus command set. Is it always possible for
the host user space to turn the I2C transaction back into the
expected SMBus transaction on the host?

quoted

quoted

Also, as I read it, a whole bus is para-virtualized to the guest, or?
Wouldn't it be better to allow just specific devices on a bus? Again, I
am kinda new to this, so I may have overlooked things.

Do you mean just allowing a single device per bus (as opposed to
having multiple devices as on a real bus), or just allowing
a particular set of client devices that can be identified using
virtio specific configuration (as opposed to relying on device
tree or similar for probing). Both of these are valid questions that
have been discussed before, but that could be revisited.

I am just thinking that a physical bus on the host may have devices that
should be shared with the guest while other devices on the same bus
should not be shared (PMIC!). I'd think this is a minimal requirement
for security. I also wonder if it is possible to have a paravirtualized
bus in the guest which has multiple devices from the host sitting on
different physical busses there. But that may be the cherry on the top.

This is certainly possible, but is independent of the implementation of
the guest driver. It's up to the host to provision the devices that
are actually passed down to the guest, and this could in theory
be any combination of emulated devices with devices connected to
any of the host's physical buses. The host may also decide to remap
the addresses of the devices during passthrough.

       Arnd
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization