need this for swapcontext(), KSE upcalls initiated from ast()
also need to save them so that we properly return the syscall
results after having had a context switch. Note that we don't
use r11 in the kernel. However, the runtime specification has
defined r8-r11 as return registers, so we put r11 in the context
as well. I think deischen@ was trying to tell me that we should
save the return registers before. I just wasn't ready for it :-)
o The EPC syscall code has 2 return registers and 2 frame markers
to save. The first (rp/pfs) belongs to the syscall stub itself.
The second (iip/cfm) belongs to the caller of the syscall stub.
We want to put the second in the context (note that iip and cfm
relate to interrupts. They are only being misused by the syscall
code, but are not part of a regular context).
This way, when the context is switched to again, we return to
the caller of setcontext(2) as one would expect.
o Deal with dirty registers on the kernel stack. The getcontext()
syscall will flush the RSE, so we don't expect any dirty registers
in that case. However, in thread_userret() we also need to save
the context in certain cases. When that happens, we are sure that
there are dirty registers on the kernel stack.
This implementation simply copies the registers, one at a time,
from the kernel stack to the user stack. NAT collections are not
dealt with. Hence we don't preserve NaT bits. A better solution
needs to be found at some later time.
We also don't deal with this in all cases in set_mcontext. No
temporay solution is implemented because it's not a showstopper.
The problem is that we need to ignore the dirty registers and we
automaticly do that for at most 62 registers. When there are more
than 62 dirty registers we have a memory "leak".
This commit is fundamental for KSE support.
7f36189f8a