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

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

[Zhenzhong Duan]

nmi isn't supported in dom0, fallback to general all cpu backtrace code.

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.

-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@oracle.com>
Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
---
 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));
+
 	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;
 	arch_trigger_all_cpu_backtrace();
 
 	return true;
-- 
1.7.3

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

[Jan Beulich]

>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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@oracle.com>
> Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
> ---
>  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;

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

[Ian Campbell]

On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
> >>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.

You'd also need to implement the upcall support for receiving NMIs,
which IIRC isn't yet done for pvops.

Ian.

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

[Zhenzhong Duan]

On 2013-04-10 00:36, Ian Campbell wrote:
> On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
>>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.
> You'd also need to implement the upcall support for receiving NMIs,
> which IIRC isn't yet done for pvops.
Hi Ian,

Could you give a suggestion on which file to change to support NMI upcall?
I compare with vMCE code, made similar change.
Use VCPUOP_send_nmi to send nmi between pvm guest vcpus, but nmi isn't 
triggered.

thanks
zduan

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

[Zhenzhong Duan]

On 2013-04-10 00:36, Ian Campbell wrote:
> On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
>>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.
> You'd also need to implement the upcall support for receiving NMIs,
> which IIRC isn't yet done for pvops.
Hi Ian,

Could you give a suggestion on which file to change to support NMI upcall?
I compare with vMCE code, made similar change.
Use VCPUOP_send_nmi to send nmi between pvm guest vcpus, but nmi isn't 
triggered.

thanks
zduan

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

[Ian Campbell]

On Wed, 2013-05-15 at 16:40 +0800, Zhenzhong Duan wrote:
> On 2013-04-10 00:36, Ian Campbell wrote:
> > On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
> >>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.
> > You'd also need to implement the upcall support for receiving NMIs,
> > which IIRC isn't yet done for pvops.
> Hi Ian,
> 
> Could you give a suggestion on which file to change to support NMI upcall?
> I compare with vMCE code, made similar change.
> Use VCPUOP_send_nmi to send nmi between pvm guest vcpus, but nmi isn't 
> triggered.

You need to register a callback with CALLBACKOP_register
CALLBACKTYPE_nmi. You also need to write the code in entry.S to receive
that callback. IIRC you also need to arrange that returning from an NMI
is always done with HYPERVISOR_iret and not optimised to a direct iret
as it can be otherwise. This is to allow the hypervisor to implement NMI
masking correctly.

The linux-2.6.18-xen.hg tree implements NMI callbacks so you may find
inspiration there, although how upstreamable that approach is I'm not
sure. In particular stealing a EFLAGS bit to force the HYPERVISOR_iret
is certain to not be acceptable upstream.

Ian.

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

[Ian Campbell]

On Wed, 2013-05-15 at 16:40 +0800, Zhenzhong Duan wrote:
> On 2013-04-10 00:36, Ian Campbell wrote:
> > On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
> >>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.
> > You'd also need to implement the upcall support for receiving NMIs,
> > which IIRC isn't yet done for pvops.
> Hi Ian,
> 
> Could you give a suggestion on which file to change to support NMI upcall?
> I compare with vMCE code, made similar change.
> Use VCPUOP_send_nmi to send nmi between pvm guest vcpus, but nmi isn't 
> triggered.

You need to register a callback with CALLBACKOP_register
CALLBACKTYPE_nmi. You also need to write the code in entry.S to receive
that callback. IIRC you also need to arrange that returning from an NMI
is always done with HYPERVISOR_iret and not optimised to a direct iret
as it can be otherwise. This is to allow the hypervisor to implement NMI
masking correctly.

The linux-2.6.18-xen.hg tree implements NMI callbacks so you may find
inspiration there, although how upstreamable that approach is I'm not
sure. In particular stealing a EFLAGS bit to force the HYPERVISOR_iret
is certain to not be acceptable upstream.

Ian.

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

[Ian Campbell]

On Mon, 2013-04-08 at 08:42 +0100, Jan Beulich wrote:
> >>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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.

You'd also need to implement the upcall support for receiving NMIs,
which IIRC isn't yet done for pvops.

Ian.

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

[Stefan Bader]

[-- Attachment #1.1: Type: text/plain, Size: 3795 bytes --]

On 08.04.2013 09:42, Jan Beulich wrote:
>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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@oracle.com>
>> Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
>> ---
>>  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;
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
> 



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

[Stefan Bader]

[-- Attachment #1: Type: text/plain, Size: 3795 bytes --]

On 08.04.2013 09:42, Jan Beulich wrote:
>>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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@oracle.com>
>> Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
>> ---
>>  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;
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
> 



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

[Jan Beulich]

>>> On 07.04.13 at 07:54, Zhenzhong Duan <zhenzhong.duan@oracle.com> 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@oracle.com>
> Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
> ---
>  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;