base-hw & the root task

Neal H. Walfield neal at ...8...
Tue Nov 5 16:36:48 CET 2013

Hi, Martin,

At Tue, 05 Nov 2013 16:27:30 +0100,
Martin Stein wrote:
> On 05.11.2013 15:59, Neal H. Walfield wrote:
> > At Tue, 05 Nov 2013 15:15:49 +0100,
> > Martin Stein wrote:
> >> The single "kernel"-thread executes solely in privileged CPU mode.
> >> Initially it disables the MMU and runs in physical address space.
> >> Before leaving privileged CPU mode the first time, it enables the
> >> MMU, using the address space of the root-task (core). Thus
> >> both, threads of the non-privileged root-task and the single "kernel"
> >> thread use the same virtual address space. For the purpose of
> >> simplification "kernel" thread and root-task threads also share data
> >> structures (e.g. raw page-tables or the interrupt-lookup table). This
> >> must be done carefully because "kernel" thread can always interrupt
> >> root-task threads and access shared objects without synchronization.
> > If I understand correctly, you are basically implementing a fail fast
> > mechanism for the root task.
> >
> > Thus, the root task can, say, walk the page tables, but to modify the
> > page tables or to switch protection domains, it traps to the kernel?
> > Similarly, I'm guessing the kernel doesn't walk any of the root task's
> > data structure (or, it does so very conservatively).
> >
> By now the "kernel" thread creates the root-task page-table
> at start-up in a way, that it contains 1:1 mappings for all physical
> regions that are ever needed by root-task or kernel. Thus root-task
> threads and the kernel thread never throw page-faults and access
> resources in virtual space via their physical addresses. This way we
> keep things simple while benefit from several performance features
> the ARM MMU provides. As a result of this, root-task never touches its
> own page table. If root-task threads want to create further page
> tables they use a syscall. During this syscall, kernel initializes the
> page table with generic stuff. After that, application-specific control
> of the new page table is in the hands of the root-task threads.

Thanks for the details!

:) Neal

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