The addition of ARM security extension (TrustZone) support to QEMU has exposed the issue of how secure resources are communicated to secure software responsible for booting the HLOS. The natural choice for communicating these details is the device tree. In the case of real hardware, the device tree supplied to the HLOS only needs to describe non-secure resources as secure software can rely on static knowledge about the hardware. This also holds true for QEMU machines modeled after actual fixed hardware configurations. However, this is not the case with QEMU's virtual machine models, such as machvirt, where the hardware configuration can change over time. In this case, secure software is dependent on QEMU's dynamically constructed device tree to describe the hardware, making it impossible for secure software to know ahead of time what is secure, non-secure, or both. Two possible approaches for handling this particular case are: 1) Create two device tree blobs; one describing the non-secure configuration and the other the full configuration. This would allow secure software to see the full hardware picture including secure resources while the non-secure world would only see the non-secure device tree configuration. The QEMU virt machine would be responsible for producing the device tree blobs. The drawbacks to this approach are: * There are 2 device trees to manage * The two DTBs will typically be almost identical. * Not possible to identify whether a device is shared or not between the secure and non-secure worlds. * Identifying device available only to the secure world require cumbersome comparison of the two device trees. * A mechanism would be needed to pass an additional device tree. 2) Modify the standard device tree blob to include annotations or modifications to describe which resources are secure or not. In this case, secure software would use the single device tree to identify the secure resources. The added information could be used by secure software to trim the device tree before passing it. Alternatively, the information could be passed on to non-secure software with the expectation that it would honor the device security. It would be crucial that any data added to the device tree adhere to existing conventions or expectations. The drawbacks to this approach are: * Additional secure state details. potentially unrecognized by current consumers, would need to be added to the device tree. * Unless a mechanism already exists, the new secure property runs the risk of breaking backwards compatibility. * Secure firmware is responsible for understanding and possibly filtering the secure device tree data. Feedback welcomed. Greg -- To unsubscribe from this list: send the line "unsubscribe devicetree-spec" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
