Hello Chian,
On 18.06.2011 01:38, Chen Tian - SISA wrote:
Hello,
After the set_pager function of a thread is called through a cpu_session, the _pager of that thread is set and used when the actual platform thread starts.
That's true at least for the implementation of the rm_session interface in core with respect to Genode/Fiasco.OC.
But there is only one thread (pager) to resolve all page faults for all threads.
This is almost true for the Fiasco.OC version of Genode. Like Norman explained in a previous mail, other platforms don't necessarily have only one pager. In the current implementation for Fiasco.OC this is true for normal Genode applications. L4Linux for example pages its VCPUs, and applications itself. A VCPU (some kind of special thread concept in recent Fiasco.OC revisions) is constructed by Genode's core process, but afterwards the capability slot containing the global pager capability gets overmapped to contain a special pager thread, which is running in the L4Linux-kernel protection-domain.
I think the pager mapping happened in platform_thread::start (platform_thread.cc:40-47). Can somebody please explain to me the meaning of this l4_task_map call?
In Fiasco.OC a pager gets assigned to a thread by choosing a capability slot in the capability space of the target protection domain, wherein the new thread will run. The capability in that slot should point to the pager thread. That why we first map the capability of the pager (which was set by the cpu_session interface first) into the capability space of the target protection domain. This is what the following snipet does:
l4_task_map(_platform_pd->native_task(), L4_BASE_TASK_CAP, l4_obj_fpage(_pager->cap().dst(), 0, L4_FPAGE_RWX), _remote_pager_cap | L4_ITEM_MAP);
After that, you instruct the kernel to use the capability slot, we just filled with life, as pager and exception-handler:
l4_thread_control_start(); l4_thread_control_pager(_remote_pager_cap); l4_thread_control_exc_handler(_remote_pager_cap); l4_thread_control_bind(_utcb, _platform_pd->native_task());
I suspect that “_remote_pager_cap” is corresponding to the global pager, but cannot find any evidence. Thanks a lot.
The '_remote_pager_cap' denotes the capability slot in the thread's capability space, whereby '_pager' is the capability slot in core's capability space, that gets mapped to '_remote_pager_cap' by l4_task_map. The '_pager' variable is set via the cpu_session interface, like you've stated before. Although, this is normally the "one global pager", Genode's design and the Fiasco.OC specific implementation is not limited to use only that one pager thread.
Best regards Stefan
Best,
Chen
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