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SubjectRe: Boot failures on Qemu due to P6_NOPS
Date
Christian Ehrhardt wrote:

> Hi,
>
> this might well be a bug in Qemu but even then, it would be nice if the
> linux kernel could do a work around.
>
> I tried to boot a current git kernel (around 2.6.26-rc8) on qemu and
> got the an invalid opcode oops on boot (full oops data below).
>
> The illegal instruction is 0x0f 0x1f 0x00 aka P6_NOP3.
>
> I have verified that this opcode gets patched in because
> apply_alternatives() or more precisely add_nops() uses P6 nops
> on this CPU type while padding after patching in an fxsave
> instruction. More precisely the code that oopses is:
>
> fxsave (%eax)
> btl $0x7,0x2(%eax)
> jae 0x804833e <main+26>
> fnclex
> nopl (%eax) <==== Faulting instruction
>
> P6 nops are used when patching because init_intel() sets X86_FEATURE_P3 for
> family 6 CPUs and X86_FEATURE_P3 in turn enables the P6 NOPS.
>
> The Qemu CPU identifies itself as follows:
>
> processor : 0
> vendor_id : GenuineIntel
> cpu family : 6
> model : 3
> model name : Pentium II (Klamath)
> stepping : 3
> cpu MHz : 1862.133
>
> I have no idea if this specific CPU type should support this specific
> instruction. If it really should this is obviously a Qemu bug but it
> might be reasonable to work around this in the linux kernel.
>
> The kernel is configured with CONFIG_M586 but without CONFIG_X86_GENERIC
> (full config upon request). Apparently setting CONFIG_X86_GENERIC works
> around the problem.
>
> ========== ooops data follows ===================================
> invalid opcode: 0000 [#1] SMP
> Modules linked in:
>
> Pid: 0, comm: swapper Not tainted (2.6.26-rc9 #1)
> EIP: 0060:[<c0102035>] EFLAGS: 00000202 CPU: 0
> EIP is at prepare_to_copy+0x1d/0x43
> EAX: c781de00 EBX: 00000000 ECX: c03f3f9c EDX: c03c23e0
> ESI: fffffff4 EDI: c03c23e0 EBP: 00000000 ESP: c03f3f08
> DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> Process swapper (pid: 0, ti=c03f2000 task=c03c23e0 task.ti=c03f2000)
> Stack: c011c653 c03f3f9c 00000000 00800b00 00000286 00000000 00000000 00000000
> 00800b00 c03f3f9c 00000000 c011d671 00000000 00000000 00000000 00000000
> c03c8cb0 00000246 c03c8a80 00000046 00000000 00000002 00000001 c014a30c
> Call Trace:
> [<c011c653>] copy_process+0x70/0xf75
> [<c011d671>] do_fork+0xab/0x19c
> [<c014a30c>] free_pages_bulk+0x23/0x1d0
> [<c0108b4a>] native_sched_clock+0x90/0xa4
> [<c03f72fd>] kernel_init+0x0/0x25d
> [<c01024b1>] kernel_thread+0x78/0x80
> [<c03f72fd>] kernel_init+0x0/0x25d
> [<c0104af0>] kernel_thread_helper+0x0/0x10
> [<c02e0d39>] rest_init+0x11/0x4b
> [<c03f7811>] start_kernel+0x2a0/0x2a3
> =======================
> Code: ff 05 0c 20 47 c0 c3 ff 0d 0c 20 47 c0 c3 89 c2 8b 40 04 f6 40 0c
> 01 74 30 8b 82 6c 02 00 00 0f ae 00 0f ba 60 02 07 73 02 db e2 <0f> 1f
> 00 90 8d b4 26 00 00 00 00 89 f6 8b 42 04 83 60 0c fe 0f
> EIP: [<c0102035>] prepare_to_copy+0x1d/0x43 SS:ESP 0068:c03f3f08
> ---[ end trace 4eaa2a86a8e2da22 ]---
> Kernel panic - not syncing: Attempted to kill the idle task!
>

This is a problem in old qemu versions which don't support multi byte NOPs.
Please check previous discussion about it at http://lkml.org/lkml/2008/5/3/60.

- Sebastian



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