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

Marco Elver
2020-08-11T07:01:25Z

On Fri, 7 Aug 2020 at 17:19, Marco Elver <elver@google.com> wrote:
> On Fri, Aug 07, 2020 at 02:08PM +0200, Marco Elver wrote:
> > On Fri, 7 Aug 2020 at 14:04, Jürgen Groß <jgross@suse.com> wrote:
> > >
> > > On 07.08.20 13:38, Marco Elver wrote:
> > > > On Fri, Aug 07, 2020 at 12:35PM +0200, Jürgen Groß wrote:
...
> > > >> I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
> > > >> sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.
> > > >
> > > > Yes, PARAVIRT_XXL doesn't make a different. When disabling
> > > > PARAVIRT_SPINLOCKS, however, the warnings go away.
> > >
> > > Thanks for testing!
> > >
> > > I take it you are doing the tests in a KVM guest?
> >
> > Yes, correct.
> >
> > > If so I have a gut feeling that the use of local_irq_save() and
> > > local_irq_restore() in kvm_wait() might be fishy. I might be completely
> > > wrong here, though.
> >
> > Happy to help debug more, although I might need patches or pointers
> > what to play with.
> >
> > > BTW, I think Xen's variant of pv spinlocks is fine (no playing with IRQ
> > > on/off).
> > >
> > > Hyper-V seems to do the same as KVM, and kicking another vcpu could be
> > > problematic as well, as it is just using IPI.
>
> I experimented a bit more, and the below patch seems to solve the
> warnings. However, that was based on your pointer about kvm_wait(), and
> I can't quite tell if it is the right solution.
>
> 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
> IRQ state AFAIK.

Just to follow-up, it'd still be nice to fix this. Suggestions?

I could send the below as a patch, but can only go off my above
hypothesis and the fact that syzbot is happier, so not entirely
convincing.

Thanks,
-- Marco

> ------ >8 ------
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 233c77d056c9..1d412d1466f0 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -797,7 +797,7 @@ static void kvm_wait(u8 *ptr, u8 val)
> if (in_nmi())
> return;
>
> - local_irq_save(flags);
> + raw_local_irq_save(flags);
>
> if (READ_ONCE(*ptr) != val)
> goto out;
> @@ -810,10 +810,10 @@ static void kvm_wait(u8 *ptr, u8 val)
> if (arch_irqs_disabled_flags(flags))
> halt();
> else
> - safe_halt();
> + raw_safe_halt();
>
> out:
> - local_irq_restore(flags);
> + raw_local_irq_restore(flags);
> }
>
> #ifdef CONFIG_X86_32