base-hw: Cpu_lazy_state and alignment constraints

[Martin Stein]

Hi Adrian,

In base-hw, there are three types of executable subjects by now: user
threads, virtual machines, and CPU-idle contexts. The CPU job is the
base class that is common to all of them. All CPU-idle contexts (as
members of the CPU objects in the CPU pool in
base-hw/src/core/kernel/cpu.cc) and the first core thread (in
'init_kernel_mp_primary') are static objects.

The other threads and all VMs get created by core via the syscalls
'new_thread' and 'new_vm' providing the target address as argument. So
the address of these objects is determined by core using the allocators
of the appropriate VM/CPU session (see '_kernel_thread' in
base-hw/src/core/include/platform_thread.h, '_platform_thread' in
base/src/core/include/cpu_session_component.h, and '_vm' in
base-hw/src/core/include/spec/*/vm_session_component.h).

So, to ensure alignment in CPU job, you would have to modify attributes
of the mentioned static instances and check the pointer arguments in the
syscall back-ends of 'new_thread' and 'new_vm'. Furthermore, the session
back-ends in core would have to be modified in a way that they allocate
new 'Cpu_thread_component's and 'Vm_session_component's aligned.

I don't know wether this is worth the effort, and besides, this would
also raise complexity of the kernel interface.

Cheers,
Martin

On 09.03.2015 15:01, Adrian-Ken Rueegsegger wrote:
> Hello,
>
> While implementing lazy FPU loading/storing for the x86 base-hw port I
> came across an issue with regards to data structure alignment. The
> implementation is similar to the one of base-hw cortex A9 and uses the
> Cpu_lazy_state class to store the FPU state, see [1].
>
> To use the fx[save|rstor] instructions, the storage area must be 16-byte
> aligned which I set as alignment of the Cpu_lazy_state class [2].
> Checking the alignment of the data structure during runtime showed
> however, that the constraint is not correctly adhered to.
> I suspect that the Kernel::Cpu_job class, which contains the lazy state
> [3], is allocated without consideration for the required alignment.
>
> I have worked around the issue by manually ensuring the correct
> alignment [4] but I consider it a hack would like to get rid of it.
>
> Since I am not quite sure how Cpu_job is allocated and what its life
> cycle looks like, I would be glad if someone could give me some pointers
> to verify my suspicion and give a suggestion on how one would go about
> ensuring the proper alignment of all instances of Cpu_lazy_state.
>
> Regards,
> Adrian
>
> [1] -
> https://github.com/codelabs-ch/genode/blob/9a2a85c807a695a91088b6ab6e0d0de422c85dc1/repos/base-hw/src/core/include/spec/x86/cpu.h#L43
> [2] -
> https://github.com/codelabs-ch/genode/blob/9a2a85c807a695a91088b6ab6e0d0de422c85dc1/repos/base-hw/src/core/include/spec/x86/cpu.h#L75
> [3] -
> https://github.com/codelabs-ch/genode/blob/hw_x86-fpu/repos/base-hw/src/core/include/kernel/cpu.h#L132
> [4] -
> https://github.com/codelabs-ch/genode/commit/996ffa6407effaa45488ddf3e41986c4fd2947ed
>
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