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

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Date: Tue, 4 Jun 2024 17:04:51 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: 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, nogikh@...gle.com, 
	tarasmadan@...gle.com
Subject: Re: [PATCH 1/4] x86/entry: Remove unwanted instrumentation in common_interrupt()

On Tue, Jun 4, 2024 at 3:45 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> common_interrupt() and friends call kvm_set_cpu_l1tf_flush_l1d(),
> which is not marked as noinstr nor __always_inline.
> So compiler outlines it and adds instrumentation to it.
> Since the call is inside of instrumentation_begin/end(),
> objtool does not warn about it.
>
> 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.
>
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>