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

From: Jürgen Groß
Date: Fri Aug 07 2020 - 08:04:28 EST

On 07.08.20 13:38, Marco Elver wrote:
On Fri, Aug 07, 2020 at 12:35PM +0200, Jürgen Groß wrote:
On 07.08.20 11:50, Marco Elver wrote:
On Fri, Aug 07, 2020 at 11:24AM +0200, Jürgen Groß wrote:
On 07.08.20 11:01, Marco Elver wrote:
On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver@xxxxxxxxxx> wrote:
On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver@xxxxxxxxxx> wrote:
On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@xxxxxxxxxxxxx wrote:
On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:
Testing my hypothesis that raw then nested non-raw
local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
below. This is at least 1 case I can think of that we're bound to hit.

/me goes ponder things...

How's something like this then?

include/linux/sched.h | 3 ---
kernel/kcsan/core.c | 62 ++++++++++++++++++++++++++++++++++++---------------
2 files changed, 44 insertions(+), 21 deletions(-)

Thank you! That approach seems to pass syzbot (also with
CONFIG_PARAVIRT) and kcsan-test tests.

I had to modify it some, so that report.c's use of the restore logic
works and not mess up the IRQ trace printed on KCSAN reports (with

I still need to fully convince myself all is well now and we don't end
up with more fixes. :-) If it passes further testing, I'll send it as a
real patch (I want to add you as Co-developed-by, but would need your
Signed-off-by for the code you pasted, I think.)

I let it run on syzbot through the night, and it's fine without
PARAVIRT (see below). I have sent the patch (need your Signed-off-by
as it's based on your code, thank you!):

With CONFIG_PARAVIRT=y (without the notrace->noinstr patch), I still
get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
it takes longer for syzbot to hit them. But I think that's expected
because we can still get the recursion that I pointed out, and will
need that patch.

Never mind, I get these warnings even if I don't turn on KCSAN
(CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
throws off IRQ state tracking. :-/

What are the settings of CONFIG_PARAVIRT_XXL and

I attached a config.

$> grep PARAVIRT .config

Anything special I need to do to reproduce the problem? Or would you be
willing to do some more rounds with different config settings?

I can only test it with syzkaller, but that probably doesn't help if you
don't already have it set up. It can't seem to find a C reproducer.

I did some more rounds with different configs.

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?

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.

BTW, I think Xen's variant of pv spinlocks is fine (no playing with IRQ

Hyper-V seems to do the same as KVM, and kicking another vcpu could be
problematic as well, as it is just using IPI.