Hi,
As once discussed with Heikki, and after a first missed attempt, here
is a patch for x86 which attempts to cope correctly with Linux (non
real-time) drivers using FPU in kernel-space.
I tested it on my laptop without drivers using FPU, and it appears to
work correctly, but it would be nice if people that had problems with
the previous attempt (if I remember correctly, on old pentiums and on
AMD processors) could test this patch.
Thanks in advance.
--
Gilles Chanteperdrix
Index: include/asm-i386/system.h
===================================================================
--- include/asm-i386/system.h (revision 1704)
+++ include/asm-i386/system.h (working copy)
@@ -69,6 +69,7 @@
/* FPU context bits for root thread. */
unsigned is_root: 1;
unsigned ts_usedfpu: 1;
+ unsigned cr0_ts: 1;
} xnarchtcb_t;
Index: include/asm-i386/bits/pod.h
===================================================================
--- include/asm-i386/bits/pod.h (revision 1704)
+++ include/asm-i386/bits/pod.h (working copy)
@@ -44,6 +44,7 @@
/* Remember the preempted Linux task pointer. */
rootcb->user_task = rootcb->active_task = current;
rootcb->ts_usedfpu = wrap_test_fpu_used(current) != 0;
+ rootcb->cr0_ts = (read_cr0() & 8) != 0;
/* So that xnarch_save_fpu() will operate on the right FPU area. */
rootcb->fpup = &rootcb->user_task->thread.i387;
}
@@ -265,12 +266,21 @@
{
struct task_struct *task = tcb->user_task;
- if (task) {
- if (!wrap_test_fpu_used(task))
+ if (!tcb->is_root) {
+ if (task) {
+ /* fpu not used or already saved by __switch_to. */
+ if (!wrap_test_fpu_used(task))
+ return;
+
+ /* Tell Linux that we already saved the state of the FPU
+ hardware of this task. */
+ wrap_clear_fpu_used(task);
+ }
+ } else {
+ if (tcb->cr0_ts ||
+ (tcb->ts_usedfpu && !wrap_test_fpu_used(task)))
return;
- /* Tell Linux that we already saved the state of the FPU
- hardware of this task. */
wrap_clear_fpu_used(task);
}
@@ -298,13 +308,14 @@
wrap_set_fpu_used(task);
}
} else {
- /* Restore state of FPU if TS_USEFPU bit was armed. */
- if (!tcb->ts_usedfpu) {
+ /* Restore state of FPU only if TS bit in cr0 was clear. */
+ if (tcb->cr0_ts) {
stts();
return;
}
- wrap_set_fpu_used(task);
+ if (tcb->ts_usedfpu)
+ wrap_set_fpu_used(task);
}
/* Restore the FPU hardware with valid fp registers from a
@@ -336,7 +347,7 @@
}
}
} else {
- if (!tcb->ts_usedfpu)
+ if (tcb->cr0_ts)
return;
xnarch_restore_fpu(tcb);
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