Re: [Xenomai] [PATCH] x86/ipipe: get FPU backup area allocated for all threads

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-10-04 10:50, Gilles Chanteperdrix wrote:
> 
> Jan Kiszka wrote:
>> On 2013-09-30 21:21, Gilles Chanteperdrix wrote:
>>> In order to be able to handle FPU faults in the Xenomai domain without
>>> switching to the root domain, the FPU backup area must be systematically
>>> allocated for all Linux tasks. Currently, the backup area is
>>> systematically
>>> allocated in arch_dup_task_struct(), but gets freed upon exec by
>>> flush_thread(). Stop freeing it in flush_thread(), there does not seem
>>> to be any risk of leaking information, as the "used_math" flag is
>>> cleared
>>> by drop_init_fpui() so that upon next fault, the context is
>>> reinitialized.
>>
>> How to reproduce the scenario? Would be good to document it in order to
>> check for regressions in future kernels.
> 
> Simply shadow the main thread, and use FPU in primary mode, without having
> first used it in secondary mode. You should get a kernel oops.
> 
> We can add a test to switchtest, the problem is that switchtest purposedly
> avoided to use FPU in the main thread before creating the other threads
> to test threads which are not created with the "used_math" flag. But we can
> use FPU after the other threads creation, right before the call to sigwait.

Yes, having an explicit test case would be great. Also important is to
avoid using a modern CPU with XSAVE support. Most of our target have
this feature now.

Jan

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