Re: [Xen-devel] [PATCH-v2] xen: Don't call arch_trigger_all_cpu_backtrace in dom0(pvm)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> wrote:
> nmi isn't supported in dom0, fallback to general all cpu backtrace code.

Since when is sending NMIs not supported, and since when is this
Dom0-specific? If you want to deal with this, you should do so
properly: Special case sending NMIs in the respective Xen specific
code (using VCPUOP_send_nmi), and carry this out in a way not
dependent upon running (un)privileged.

> Without fix, on xAPIC system, doing sysrq+l, no backtrace is showed,
> as xen_send_IPI_all is called and it doesn't support nmi vector.
> 
> On x2APIC enabled system, got NULL pointer dereference as below.
> 
>     SysRq : Show backtrace of all active CPUs
>     BUG: unable to handle kernel NULL pointer dereference at           (null)
>     IP: [<ffffffff8125e3cb>] memcpy+0xb/0x120
>     Call Trace:
>      [<ffffffff81039633>] ? __x2apic_send_IPI_mask+0x73/0x160
>      [<ffffffff8103973e>] x2apic_send_IPI_all+0x1e/0x20
>      [<ffffffff8103498c>] arch_trigger_all_cpu_backtrace+0x6c/0xb0
>      [<ffffffff81501be4>] ? _raw_spin_lock_irqsave+0x34/0x50
>      [<ffffffff8131654e>] sysrq_handle_showallcpus+0xe/0x10
>      [<ffffffff8131616d>] __handle_sysrq+0x7d/0x140
>      [<ffffffff81316230>] ? __handle_sysrq+0x140/0x140
>      [<ffffffff81316287>] write_sysrq_trigger+0x57/0x60
>      [<ffffffff811ca996>] proc_reg_write+0x86/0xc0
>      [<ffffffff8116dd8e>] vfs_write+0xce/0x190
>      [<ffffffff8116e3e5>] sys_write+0x55/0x90
>      [<ffffffff8150a242>] system_call_fastpath+0x16/0x1b
> 
> That's because apic points to apic_x2apic_cluster or apic_x2apic_phys
> but the basic element like cpumask isn't initialized.

That's of course a bug on its own, fixing of which would go under
a suitable subject/title.

> -v2: Mask x2APIC feature in pvm to avoid overwrite of apic pointer,
>      update commit message per Konrad's suggestion.
> 
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> Tested-by: Tamon Shiose <tamon.shiose@xxxxxxxxxx>
> ---
>  arch/x86/xen/enlighten.c |    3 +++
>  include/linux/nmi.h      |    2 ++
>  2 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index c8e1c7b..12b0718 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -386,6 +386,9 @@ static void __init xen_init_cpuid_mask(void)
>               cpuid_leaf1_edx_mask &=
>                       ~((1 << X86_FEATURE_APIC) |  /* disable local APIC */
>                         (1 << X86_FEATURE_ACPI));  /* disable ACPI */
> +
> +     cpuid_leaf1_ecx_mask &= ~(1 << (X86_FEATURE_X2APIC % 32));
> +

Bottom line - while this part may be fine (under a different title), ...

>       ax = 1;
>       cx = 0;
>       xen_cpuid(&ax, &bx, &cx, &dx);
> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
> index db50840..b845757 100644
> --- a/include/linux/nmi.h
> +++ b/include/linux/nmi.h
> @@ -32,6 +32,8 @@ static inline void touch_nmi_watchdog(void)
>  #ifdef arch_trigger_all_cpu_backtrace
>  static inline bool trigger_all_cpu_backtrace(void)
>  {
> +     if (xen_domain())
> +             return false;

... this part clearly isn't.

Jan

>       arch_trigger_all_cpu_backtrace();
>  
>       return true;



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