The KVM API has an ioctl KVM_ARM_PREFERRED_TARGET which we can use to create virtual CPUs of whatever the host CPU's "preferred" type is (typically "same as host"). This allows userspace to create a VCPU for best performance without having to have specific knowledge about that CPU. However there's a slight wrinkle when userspace comes to create the device tree to pass to the guest kernel. The device tree binding for v7 says that the cpu nodes in the device tree must have a "compatible" string, and the only valid values it lists all nail things down to an exact CPU model. Requiring userspace to have an explicit mapping table of target values to dtb compat strings defeats the point of userspace not having to have hardcoded knowledge of all CPU types. This problem doesn't exist for v8, because there the compat string is always "arm,arm-v8", indicating that the kernel doesn't care beyond "this is a v8 CPU of some kind". In fact I'm told that for v7 the kernel doesn't care either (both because it needs to know about the CPU type much earlier than it has access to information in the dtb and also because this is all probeable via ID registers anyhow). So the obvious solution would be for userspace to pass a 'compatible' string "arm,arm-v7" (which the kernel will ignore anyway). Does anybody see a problem with this approach? If it seems like a good plan, how do we go about getting it put into the official device tree binding docs? thanks -- PMM -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
