On Wed, Jan 21, 2015 at 12:05 PM, Christoffer Dall <christoffer.dall@xxxxxxxxxx> wrote: > On Wed, Jan 21, 2015 at 7:01 PM, Greg Bellows <greg.bellows@xxxxxxxxxx> wrote: >> Thanks Mark, comments inline. >> >> On Wed, Jan 21, 2015 at 10:29 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote: >>> On Tue, Jan 20, 2015 at 07:15:51PM +0000, Greg Bellows wrote: >>>> 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. >>> >>> When you say this changes over time, I assume you mean for the lifetime >>> of the project, as opposed to the lifetime of a running instance? i.e. >>> non-probeable devices are not dynamically injected to a running system. >>> >> >> Correct, the project. The QEMU machvirt device can be adapted over >> time to include additional resources. >> >>>> 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. >>> >>> I'm not sure I follow why such identification is necessary? >>> >> >> Identifying secure resources would be necessary in cases where secure >> resources are removed prior to passing to the next level. In some >> cases, it may also be an indicator that the resource needs >> initialization by secure SW. >> >>>> * 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. >>> >>> This approach assumes assumes that the secure and non-secure physical >>> address spaces are identical bar some portions being masked out on the >>> non-secure side. This is not necessarily the case. >>> >>> Architecturally the secure and non-secure physical address spaces are >>> entiorely separate, and do not necessarily mirror each other. >>> >> >> Considering that the secure world can access both the secure and >> non-secure address spaces, it seems imperative that the resource >> include information on whether it is secure or non-secure so the >> correct address can be used. >> >>> It's entirely valid for some devices/RAM to only exist in one of the >>> address spaces (we typically see secure-only devices, but >>> non-secure-only devices are also possible). It's also entirely valid for >>> the same device to be mapped at different addresses in each address >>> space (e.g. the same UART could be mapped at both S:0xffff0000 and also >>> at NS:0xcccc0000 and nowhere else in either address space). >>> >> >> This certainly throws a wrench into things. Short of adding a >> parallel "non-secure" indicator, approach #2 would fall short. >> >>> So approach (2) does not fit the architecture generally. From what I >>> recall of previous discussions, we eventually figured out that you >>> either need separate trees or a higher level container to address the >>> secure vs nonsecure split. >> >> I was unaware of past discussions but it sounds like the two DT >> approach was decidedly more preferable. It also sounds like the 2 DT >> approach must include 2 full DTs rather than a secure-only and >> non-secure only. The latter approach would not allow non-secure only >> resources to be distinguishable from shared non-secure resources. >> > Using separate DTs, would there be any way for secure firmware to > identify a device mapped at S:0xFOO and NS:0xBAR as being actually the > same device? > Maybe alias nodes can be used that are common between the two? > -Christoffer -- 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
