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

[Stefan Bader]

On 08.04.2013 09:42, Jan Beulich wrote:
>>>> 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.

FWIW, it seems different because PVM domU's end up using noop_apic, only dom0
seems to use the xen replacements for sending IPIs through apic->...
And the last time I looked there was no maping from NMI_VECTOR to a Xen
vector/handler.

> 
>> 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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