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

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Message-ID: <3f15814c-de9c-4aa6-b56d-82761bb6520a@intel.com>
Date: Tue, 4 Jun 2024 09:00:59 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Dmitry Vyukov <dvyukov@...gle.com>, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, syzkaller@...glegroups.com,
 elver@...gle.com, glider@...gle.com, nogikh@...gle.com,
 tarasmadan@...gle.com, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] x86/entry: Remove unwanted instrumentation in
 common_interrupt()

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?