DomU panic during boot with recent timer changes (+ workaround)

[Colin Percival]

Hi all,

Following mav's timer refactoring (r212541) I'm seeing a panic during boot:

> 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

It looks like the privileged instruction fault is being triggered by mwait.
Setting machdep.idle_mwait=0 prevents this panic; I'm not sure if Xen is
supposed to handle mwait or not, so I don't know if mwait should be disabled
unconditionally for Xen/PV kernels, but maybe someone here knows more than I
do. :-)

-- 
Colin Percival
Security Officer, FreeBSD | freebsd.org | The power to serve
Founder / author, Tarsnap | tarsnap.com | Online backups for the truly paranoid