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Re: [Xen-devel] [ARM] SMC (and HVC) handling in hypervisor

Hi Tamas,

On 13/02/17 16:20, Tamas K Lengyel wrote:
On Fri, Feb 10, 2017 at 5:14 PM, Volodymyr Babchuk
<vlad.babchuk@xxxxxxxxx> wrote:

This e-mail is sort of follow-up to the two threads: [1] (my thread
about TEE interaction) and [2] (Edgar's thread regarding handling SMC
calls in platform_hvc). I want to discuss more broad topic there.

Obviously, there are growing number of SMC users and current state of
SMC handling in Xen satisfies nobody. My team wants to handle SMCs in
secure way, Xilinx wants to forward some calls directly to Secure
Monitor, while allowing to handle other in userspace, etc.

My proposition is to gather all requirements to SMC (and HVC) handling
in one place (e.g. in this mail thread). After we' will have clear
picture of what we want, we will be able to develop some solution,
that will satisfy us all. At least, I hope so :)

Also I want to remind, that there are ARM document called "SMC Calling
Convention" [3]. According to it, any aarch64 hypervisor "must
implement the Standard Secure and Hypervisor Service calls". At this
moment XEN does not conform to this.

So, lets get started with the requirements:
0. There are no much difference between SMC and HVC handling (at least
according to SMCCC).
1. Hypervisor should at least provide own UUID and version while
called by SMC/HVC
2. Hypervisor should forward some calls from dom0 directly to Secure
Monitor (Xilinx use case)
3. Hypervisor should virtualize PSCI calls, CPU service calls, ARM
architecture service calls, etc.
4. Hypervisor should handle TEE calls in a secure way (e.g. no
untrusted handlers in Dom0 userspace).
5. Hypervisor should support multiple TEEs (at least at compilation time).
6. Hypervisor should do this as fast as possible (DRM playback use case).
7. All domains (including dom0) should be handled in the same way.
8. Not all domains will have right to issue certain SMCs.
9. Hypervisor will issue own SMCs in some cases.

10. Domains on which the monitor privileged call feature is enabled
(which is by default disabled for all domains) should not be able to
issue SMCs such that it reaches the firmware directly. Xen should not
bounce such calls to the firmware on behalf of the domain. Xen should
not alter the state of the domain automatically (ie. incrementing PC).
These calls should be exclusively transfered to the monitor subscriber
for further processing. HVC calls need not be included in the monitor
forwarding as long as the HVC call can be governed by XSM.

This should not be a strong requirement. Whilst in your use case you want to forward all the SMCs to the monitor app, there are use case where Xen would need to emulate SMCs on the behalf of the guest. For instance see PSCI (arch/arm/vpsci.c).

Another valid use case is Xen handling power management for device assigned to the guest and having the monitor app acting as a "Trusted App".

Regarding the HVC call governed by XSM. I think this is the wrong way to g. As it was mentioned a couple of time HVC is a valid conduit for Secure monitor call. You should not deny them on the basis that your monitor app is not able to handle it properly. A better way would be to have a list of Secure Monitor Call (HVC/SMC) that should be forwarded to the monitor app.


Julien Grall

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