Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

Jürgen Groß
2020-08-11T08:18:55Z

On 11.08.20 10:12, Peter Zijlstra wrote:
> On Tue, Aug 11, 2020 at 09:57:55AM +0200, Jürgen Groß 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