[PATCH v6] Add virtio SCMI device specification

Russell King - ARM Linux admin linux at armlinux.org.uk
Tue Feb 16 11:57:05 EST 2021

On Tue, Feb 16, 2021 at 04:48:30PM +0000, Souvik Chakravarty wrote:
> > From: Russell King - ARM Linux admin <linux at armlinux.org.uk>
> > Sent: Tuesday, February 16, 2021 4:12 PM
> > I'm not too familiar with SCMI, but I think this question is worth asking...
> > 
> > If the SCMI protocol can be used to control system level power management,
> > and if the intention is to expose this firmware interface to virtualised guests,
> > what prevents a guest from controlling the power settings for stuff it should
> > not have access to?
> > 
> > For example, if it's possible to tell the system to power down a critical host
> > component through SCMI, what would prevent a guest requesting that
> > critical component from having its power cut?
> Short summary:
> SCMI as a protocol has built in requirements where only the resources (specific clock, sensor etc.)
> which are specifically needed by a VM are exposed to it. Resources are mapped by Identifiers and if
> the VM tries to access an identifier which it does not have access to, the SCMI backend
> can simply ignore or return DENIED. At no point is direct access to any power mgmt. hardware
> granted to any VM, nor is a VM supposed to have global access to all system resources.
> There is always a firmware backend which controls the hardware and services
> SCMI command requests from agents/guests, after due validation. 
> The SCMI device/firmware which implements the SCMI backend, is responsible for implementing these
> resource isolation guarantees.

You seem to be saying the SCMI firmware itself is responsible for
implementing this. Given what I've seen from vendors in ATF, this
does not leave me with much confidence that there will be sufficient
security. It concerns me more when you say that the "backend" is
responsible for making these decisions. This doesn't sound good to me.

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