Linux Virtualization - Aug 2020 - [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

On 11.08.20 09:41, Peter Zijlstra wrote:
> On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
> 
>> My hypothesis here is simply that kvm_wait() may be called in a place
>> where we get the same case I mentioned to Peter,
>>
>> 	raw_local_irq_save(); /* or other IRQs off without tracing */
>> 	...
>> 	kvm_wait() /* IRQ state tracing gets confused */
>> 	...
>> 	raw_local_irq_restore();
>>
>> and therefore, using raw variants in kvm_wait() works. It's also
safe
>> because it doesn't call any other libraries that would result in
corrupt
> 
> Yes, this is definitely an issue.
> 
> Tracing, we also musn't call into tracing when using raw_local_irq_*().
> Because then we re-intoduce this same issue all over again.
> 
> Both halt() and safe_halt() are more paravirt calls, but given we're in
> a KVM paravirt call already, I suppose we can directly use native_*()
> here.
> 
> Something like so then... I suppose, but then the Xen variants need TLC
> too.
Just to be sure I understand you correct:

You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
called by those should gain the "notrace" attribute, right?

I am not sure why the kick variants need it, though. IMO those are
called only after the lock has been released, so they should be fine
without notrace.

And again: we shouldn't forget the Hyper-V variants.


Juergen

On Tue, Aug 11, 2020 at 09:57:55AM +0200, J?rgen Gro?
wrote:
> On 11.08.20 09:41, Peter Zijlstra wrote:
> > On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
> > 
> > > My hypothesis here is simply that kvm_wait() may be called in a
place
> > > where we get the same case I mentioned to Peter,
> > > 
> > > 	raw_local_irq_save(); /* or other IRQs off without tracing */
> > > 	...
> > > 	kvm_wait() /* IRQ state tracing gets confused */
> > > 	...
> > > 	raw_local_irq_restore();
> > > 
> > > and therefore, using raw variants in kvm_wait() works. It's
also safe
> > > because it doesn't call any other libraries that would result
in corrupt
> > 
> > Yes, this is definitely an issue.
> > 
> > Tracing, we also musn't call into tracing when using
raw_local_irq_*().
> > Because then we re-intoduce this same issue all over again.
> > 
> > Both halt() and safe_halt() are more paravirt calls, but given
we're in
> > a KVM paravirt call already, I suppose we can directly use native_*()
> > here.
> > 
> > Something like so then... I suppose, but then the Xen variants need
TLC
> > too.
> 
> Just to be sure I understand you correct:
> 
> You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
> called by those should gain the "notrace" attribute, right?
> 
> I am not sure why the kick variants need it, though. IMO those are
> called only after the lock has been released, so they should be fine
> without notrace.
The issue happens when someone uses arch_spinlock_t under
raw_local_irq_*().
> And again: we shouldn't forget the Hyper-V variants.
Bah, my grep failed :/ Also *groan*, that's calling apic->send_IPI().

On 11.08.20 10:12, Peter Zijlstra wrote:
> On Tue, Aug 11, 2020 at 09:57:55AM +0200, J?rgen Gro? wrote:
>> On 11.08.20 09:41, Peter Zijlstra wrote:
>>> On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
>>>
>>>> My hypothesis here is simply that kvm_wait() may be called in a
place
>>>> where we get the same case I mentioned to Peter,
>>>>
>>>> 	raw_local_irq_save(); /* or other IRQs off without tracing */
>>>> 	...
>>>> 	kvm_wait() /* IRQ state tracing gets confused */
>>>> 	...
>>>> 	raw_local_irq_restore();
>>>>
>>>> and therefore, using raw variants in kvm_wait() works. It's
also safe
>>>> because it doesn't call any other libraries that would
result in corrupt
>>>
>>> Yes, this is definitely an issue.
>>>
>>> Tracing, we also musn't call into tracing when using
raw_local_irq_*().
>>> Because then we re-intoduce this same issue all over again.
>>>
>>> Both halt() and safe_halt() are more paravirt calls, but given
we're in
>>> a KVM paravirt call already, I suppose we can directly use
native_*()
>>> here.
>>>
>>> Something like so then... I suppose, but then the Xen variants need
TLC
>>> too.
>>
>> Just to be sure I understand you correct:
>>
>> You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
>> called by those should gain the "notrace" attribute, right?
>>
>> I am not sure why the kick variants need it, though. IMO those are
>> called only after the lock has been released, so they should be fine
>> without notrace.
> 
> The issue happens when someone uses arch_spinlock_t under
> raw_local_irq_*().
Ah, okay.
> 
>> And again: we shouldn't forget the Hyper-V variants.
> 
> Bah, my grep failed :/ Also *groan*, that's calling
apic->send_IPI().
I've seen that, too. :-(


Juergen

On 11.08.20 10:12, Peter Zijlstra wrote:
> On Tue, Aug 11, 2020 at 09:57:55AM +0200, J?rgen Gro? wrote:
>> On 11.08.20 09:41, Peter Zijlstra wrote:
>>> On Fri, Aug 07, 2020 at 05:19:03PM +0200, Marco Elver wrote:
>>>
>>>> My hypothesis here is simply that kvm_wait() may be called in a
place
>>>> where we get the same case I mentioned to Peter,
>>>>
>>>> 	raw_local_irq_save(); /* or other IRQs off without tracing */
>>>> 	...
>>>> 	kvm_wait() /* IRQ state tracing gets confused */
>>>> 	...
>>>> 	raw_local_irq_restore();
>>>>
>>>> and therefore, using raw variants in kvm_wait() works. It's
also safe
>>>> because it doesn't call any other libraries that would
result in corrupt
>>>
>>> Yes, this is definitely an issue.
>>>
>>> Tracing, we also musn't call into tracing when using
raw_local_irq_*().
>>> Because then we re-intoduce this same issue all over again.
>>>
>>> Both halt() and safe_halt() are more paravirt calls, but given
we're in
>>> a KVM paravirt call already, I suppose we can directly use
native_*()
>>> here.
>>>
>>> Something like so then... I suppose, but then the Xen variants need
TLC
>>> too.
>>
>> Just to be sure I understand you correct:
>>
>> You mean that xen_qlock_kick() and xen_qlock_wait() and all functions
>> called by those should gain the "notrace" attribute, right?
>>
>> I am not sure why the kick variants need it, though. IMO those are
>> called only after the lock has been released, so they should be fine
>> without notrace.
> 
> The issue happens when someone uses arch_spinlock_t under
> raw_local_irq_*().
> 
>> And again: we shouldn't forget the Hyper-V variants.
> 
> Bah, my grep failed :/ Also *groan*, that's calling
apic->send_IPI().
In case you don't want to do it I can send the patch for the Xen
variants.


Juergen