Hi,
I forgot to mention that my OS is running simultaneously with L4Linux.
In this case, it seems that Thread::transfer_fpu() becomes a no-op -- since the vcpu task for L4Linux has the FPU, but is the owner of FPU, and my OS doesn't. Therefore my OS cannot grant my user-level task the FPU since it doesn't have it.
However, my OS can grant the FPU to my user-level task, once I insert an floating-point instruction right before trying to grant my user-level process the FPU. In this case my OS magically becomes the owner of the FPU, and everything works smoothly.
What I really don't understand is that how this "magic" works. First, both L4Linux and my OS are started independently by ned:
(1) If FPU ownership can only be transferred via exception #7 and IPC, I don't understand why my OS can grab the FPU transparently, simply by executing an floating-point instruction.
(2) If there's some transparent way that an process can get an FPU, then I don't understand why my user-level process can't grab the FPU transparently. Maybe this is because that my OS is a non-alien task but my user-level task is an alien?
I really appreciate if you can explain it a little bit --- I'm really appreciated if you can share your idea of what would be the correct way of implementing it.
~Haohui
On Tue, Apr 10, 2012 at 5:07 PM, Adam Lackorzynski < adam@os.inf.tu-dresden.de> wrote:
On Sun Apr 08, 2012 at 17:22:21 -0500, Mai, Haohui wrote:
I'm writing a new OS kernel atop of L4Fiasco.OC. I'm having some difficulties to let my OS support user-level processes that uses floating-point instruction.
So far, My OS closely follows L4Linux, it creates a new task for each user-level process, and marks as all threads of the task as alien thread
to
capture all page faults and exceptions.
My OS works fines with page fault and syscalls. However, I'm confused
with
what I should do with floating-point exception (exception #7), which is issued to my kernel at the first time it tries to perform floating-point arithmetic.
I tried to return to the user-level process with an empty message ( l4_msgtag_t tag = l4_msgtag(0, 0, 0, 0) ), or an empty message with L4_MSGTAG_TRANSFER_FPU (l4_msgtag_t tag = l4_msgtag(0, 0, 0, L4_MSGTAG_TRANSFER_FPU) ), but I had no luck.
The user-level process immediately calls "int $0x3" and stops.
I read through L4Linux's code and it seems it's doing similar thing,
except
that it did initialize the FPU state before asking L4 to transfer the FPU state to user-level process.
I do not see anything (obvious) wrong. Can you pinpoint where the int3 is?
Adam
Adam adam@os.inf.tu-dresden.de Lackorzynski http://os.inf.tu-dresden.de/~adam/
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