kern/38663: Kernel preemption can't be enabled on x86 because of amd64 FPU handling

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>Number:         38663
>Category:       kern
>Synopsis:       Kernel preemption can't be enabled on x86 because of amd64 FPU 
>handling
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 15 07:45:00 +0000 2008
>Originator:     Andrew Doran
>Release:        4.99.62
>Organization:
The NetBSD Project
>Environment:
amd64
>Description:
Kernel preemption works well on i386 but cannot be enabled by default
because the amd64 port lacks the needed changes to the lazy FPU context
switching code.

>How-To-Repeat:
Code inspection, testing.
>Fix:
Implement the needed changes for amd64 FPU code.

I have a diff that does the needful, but it causes occasional hangs on
the systems that I have tried it on.