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/9a2a85c807a695a91088b6ab6e0d0de42... [2] - https://github.com/codelabs-ch/genode/blob/9a2a85c807a695a91088b6ab6e0d0de42... [3] - https://github.com/codelabs-ch/genode/blob/hw_x86-fpu/repos/base-hw/src/core... [4] - https://github.com/codelabs-ch/genode/commit/996ffa6407effaa45488ddf3e41986c...
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