Share a variable of Core with other components

[Stefan Kalkowski]

Hi Jaemin,

On Fri, Feb 02, 2018 at 11:28:53AM +0900, Jaemin Park wrote:
> Hi,
> 
> I'd like to ask a question about a way to "share a variable of Core with
> other components".
> 
> I'm using i.MX53 QSB, so this question is based on 'base-hw' implementation.
> 
> Suppose that 'Core' has a variable "A" and makes it visible to other
> components like 'Init' or else.
> 
> For this, I added a routine to use a Dataspace_capability for "A" in main()
> of 'Core' as follows:
> *[repos/base/src/core/main.cc]*
> *Dataspace_capability ds = env()->ram_session()->alloc(4096);*
> *unsigned char *local_addr = env()->rm_session()->attach(ds);*
> 
> However, I faced a difficulty to implement an RPC to delegate(share) the
> Dataspace_capability to other components.
> 
> Is there any simple or proper way to share "A" of 'Core' with other
> components?
> If possible, please give me an example.

Well, in general there are only few examples where core 'shares'
memory with other components. Due to core being the security critical
kernel component, it is a bad idea to widen the interface to it too
much. Apart from this disclaimer, there are some information needed to
be exported from core to specific components, e.g., platform
information like ACPI etc. from the booting infrastructure exported to
the platform driver. Those information are shared in a one-way
read-only fashion. Therefore, it is enough to create a dataspace
within core, and export it as a ROM module using a dedicated name for
it. The user-level component can open that dataspace using the normal
ROM session interface of core.

If you want to share information bi-directional this approach
obviously is no way. On the other hand it is not recommended at all to
import data into the kernel beyond the existing system calls.
An exception is the VM session interface of base-hw's core, which
exports a dataspace to the VMM component used to reflect the machine
register set of the VM under control of the VMM. Those register
contents can be altered by the VMM, e.g., after doing some device
emulation, and is taken by the kernel to reload the registers when
switching to the VM again. Of course, when sharing data in such a way
both sides need to synchronize before accessing the shared data.
Because core/kernel cannot block on any user-level component, signals
are used to hint the VMM about a change of the VM state, whereby the
VMM calls core via IPC (VM session interface) to mark that the VM
state handling has finished, and the VM is re-runnable.

I hope this clarifies your question. If not you may explain your
use-case in more details.

Best regards
Stefan

> 
> Any comment comments would be highly appreciated.
> 
> JaeminPark

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-- 
Stefan Kalkowski
Genode labs

https://github.com.skalk | https://genode.org