linux-kernel - Re: [PATCH 1/4] x86/entry: Remove unwanted instrumentation in common

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Date: Tue, 4 Jun 2024 21:38:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	syzkaller@...glegroups.com, elver@...gle.com, glider@...gle.com,
	nogikh@...gle.com, tarasmadan@...gle.com
Subject: Re: [PATCH 1/4] x86/entry: Remove unwanted instrumentation in
 common_interrupt()

On Tue, Jun 04, 2024 at 09:00:59AM -0700, Dave Hansen wrote:
> On 6/4/24 06:45, Dmitry Vyukov wrote:
> > The manifestation is that KCOV produces spurious coverage
> > in kvm_set_cpu_l1tf_flush_l1d() in random places because
> > the call happens when preempt count is not yet updated
> > to say that we are in an interrupt.
> > 
> > Mark kvm_set_cpu_l1tf_flush_l1d() as __always_inline and move
> > out of instrumentation_begin/end() section.
> > It only calls __this_cpu_write() which is already safe to call
> > in noinstr contexts.
> 
> I've internalized the main rules around noinstr to basically be: Only
> call noinstr functions before begin_instrumentation().  Second, try to
> minimize the amount of noinstr code.
> 
> This patch seems to be adding another rule which is that all code before
> preempt_count manipulation needs to be noinstr.
> 
> _Is_ that a new rule, or was it something I was missing?

Specifically, the problem here appears to be that the instrumentation
cannot correctly identify the context because the HARDIRQ_MASK bits
aren't yet set in preempt_count.

So the preempt_count manipulations as such are not the problem, but the
fact that we call into instrumentation code that seems to rely on using
the preempt_count to determine context is.

Does that clarify?