kern/152228: Xen/PV panic with machdep.idle_mwait=1

[Colin Percival]

>Number:         152228
>Category:       kern
>Synopsis:       Xen/PV panic with machdep.idle_mwait=1
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Nov 14 05:10:04 UTC 2010
>Closed-Date:
>Last-Modified:
>Originator:     Colin Percival
>Release:        FreeBSD 9.0-CURRENT
>Organization:
>Environment:

FreeBSD HEAD ca. 2010-11-12, XEN kernel.

This bug does not occur in a 2010-09-13 tree (aka. prior to mav's
timer refactoring).

>Description:

Kernel panic:
> Fatal trap 1: privileged instruction fault while in kernel mode
> cpuid = 0; apic id = 00
> instruction pointer     = 0x21:0xc031ac5c
> stack pointer           = 0x29:0xc2719c84
> frame pointer           = 0x29:0xc2719c88
> code segment            = base 0x0, limit 0xf9800, type 0x1b
>                         = DPL 1, pres 1, def32 1, gran 1
> processor eflags        = interrupt enabled, resume, IOPL = 0
> current process         = 11 (idle: cpu0)
> trap number             = 1
> panic: privileged instruction fault
> cpuid = 0
> KDB: stack backtrace:
> #0 0xc0106057 at kdb_backtrace+0x47
> #1 0xc00d3c47 at panic+0x117
> #2 0xc032efa3 at dblfault_handler+0x3c3
> #3 0xc032f8ad at trap+0x5ed
> #4 0xc03150bb at alltraps+0x1b
> #5 0xc031acb0 at cpu_idle+0x30
> #6 0xc00f9133 at sched_idletd+0x263
> #7 0xc00aa7a8 at fork_exit+0xb8
> #8 0xc0315134 at fork_trampoline+0x8
> Uptime: 1s

The privileged instruction fault is being triggered by mwait.

>How-To-Repeat:
>Fix:

Setting machdep.idle_mwait=0 prevents this panic.
>Release-Note:
>Audit-Trail:
>Unformatted: