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

[Jürgen Groß]

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