Re: [Xen-devel] [RFC v2 3/6] xen/arm: Allow platform_hvc to handle guest SMC calls

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxx>]

On Wed, Feb 08, 2017 at 06:29:13PM +0100, Edgar E. Iglesias wrote:
> On Wed, Feb 08, 2017 at 04:58:44PM +0000, Julien Grall wrote:
> > Hi Tamas,
> > 
> > On 08/02/17 16:40, Tamas K Lengyel wrote:
> > >On Wed, Feb 8, 2017 at 1:31 AM, Edgar E. Iglesias
> > ><edgar.iglesias@xxxxxxxxxx <mailto:edgar.iglesias@xxxxxxxxxx>> wrote:
> > >    On Tue, Feb 07, 2017 at 05:24:03PM -0700, Tamas K Lengyel wrote:
> > >    > On Tue, Feb 7, 2017 at 1:51 PM, Julien Grall <julien.grall@xxxxxxx
> > >    <mailto:julien.grall@xxxxxxx>> wrote:
> > >    I considered this approach a bit but it has several problems IMO.
> > >    These may not be unsolvable or even problems for monitoring but
> > >    they do introduce complexity into the solution.
> > >
> > >    1. Some SMC calls may depend on the core they are issued from.
> > >       If taking a detour to dom0, this becomes messy to guarantee.
> > >
> > >    2. Overall complexity increases very significantly and it becomes
> > >       quite hard to follow/review how these calls get handled.
> > >       In particular once you consider solving #1.
> > >
> > >    3. There are certain calls that perhaps not even dom0 should have
> > >       direct access to. This means that Xen may need to filter some of
> > >       them anyway.
> > >
> > >    Some examples may be:
> > >
> > >    SMC calls:
> > >    * To directly turn off or suspend cores
> > >    * To turn off DDR or RAMs that Xen is using
> > >    * To a solution specific Trusted OS pinned to a specific core
> > >    * For PSCI
> > >    * Etc
> > >
> > >    I would prefer if we could find a way for monitoring to play nicely
> > >    with Xen implementing the SMC mediators.
> > >    Perhaps we could allow calls that Xen consumes to be 
> > > monitored/inspected
> > >    but not modified. Or there might be other ways.
> > >
> > >    Best regards,
> > >    Edgar
> > >
> > >
> > >Hi Edgar,
> > >certainly there are many cases where the system would become very
> > >complex when there is functionality like what you describe in the TZ
> > >that needs to be made accessible via SMC. The setup I described is
> > >experimental only, and the underlying assumption is that the TZ is
> > >working jointly with the monitor application (ie. both are aware of each
> > >other). So it is really not intended to work with just any firmware.
> > 
> > How do you expect TrustZone to work with the monitor application? If you
> > think about modifying Trustzone, it seems a requirement difficult to achieve
> > as some Trusted OS are proprietary or difficult to replace on a phone.
> > 
> > >
> > >So I think for the sake of reducing complexity, having the monitor
> > >system be exclusive when enabled should make everyone's life simpler.
> > >Having a passive monitoring mode as you suggest is certainly an option,
> > >although it should not be the only option, exclusive routing of SMCs
> > >through monitor applications should still be available to be configured
> > >by the user. Since I really don't know of any usecases where passive
> > >monitoring of SMCs is required, I don't think we should go that route.
> > 
> > I see the SMC trap similar to a register trap. The monitor app will look at
> > the register value and potentially modify it. What would be the issue to do
> > the same for SMC?
> 
> Yes, I think this would work if we can keep the SMC processing on the same
> core (after it's been accepted by monitor filters). If not accepted by
> filters, we'd just ignore the SMC processing and return registers provided
> by the monitor (or something along those lines).
> 
> 
> > I think a such model would fit the requirement for everyone. The monitor app
> > can filter if necessary, and Xen would handle the mediation between multiple

Any ideas on how we can support monitor filtering while at the same time
having Xen processing calls? How do we wait for a response from the monitor
and then (if allowed) issue the SMC from the right core?

Or we could make these approaches mutually exclusive.
If platform_hvc() consumes an SMC, it's considered part of the Xen API.
Visible but not filterable by a monitor.

Platforms can then dictate which SMC calls are better handled within Xen and
which ones can be exposed to dom0 user-space.

In addition, there could be a hypercall to disable platform specific handling
in Xen alltogether for a given guest. Then everything goes to dom0 user-space.

It's a little messy...


> > guests. I would also recommend to read the thread about OP-TEE support in
> > Xen (see [1]).
> >

BTW, my comment below was regarding the interrupts discussion in the OP-TEE
thread, in case it seemed totally off-topic :-)

> 
> We have a similar issue with interrupts from Firmware. I've not implemented
> this part in this series but at some point I'm going to have to.
> 
> Essentially I'm looking at handling an IPI in Xen and forwarding an SGI or
> a virtual inject of the IPI to the guest that was targeted. Future work...
> 
> Cheers,
> Edgar
> 
> > Cheers,
> > 
> > [1]
> > https://lists.xenproject.org/archives/html/xen-devel/2016-11/msg02220.html
> > 
> > -- 
> > Julien Grall

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