Re: [PATCH v4 4/5] arm64: mte: Enable async tag check fault

From: Catalin Marinas <catalin.marinas@arm.com>
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
	Will Deacon <will@kernel.org>, Dmitry Vyukov <dvyukov@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Branislav Rankov <Branislav.Rankov@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v4 4/5] arm64: mte: Enable async tag check fault
Date: Tue, 19 Jan 2021 14:34:13 +0000	[thread overview]
Message-ID: <20210119143412.GD17369@gaia> (raw)
In-Reply-To: <20210118183033.41764-5-vincenzo.frascino@arm.com>

On Mon, Jan 18, 2021 at 06:30:32PM +0000, Vincenzo Frascino wrote:
>  static void update_sctlr_el1_tcf0(u64 tcf0)
>  {
>  	/* ISB required for the kernel uaccess routines */
> @@ -235,6 +273,15 @@ void mte_thread_switch(struct task_struct *next)
>  	/* avoid expensive SCTLR_EL1 accesses if no change */
>  	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
>  		update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
> +
> +	/*
> +	 * Check if an async tag exception occurred at EL1.
> +	 *
> +	 * Note: On the context switch path we rely on the dsb() present
> +	 * in __switch_to() to guarantee that the indirect writes to TFSR_EL1
> +	 * are synchronized before this point.
> +	 */
> +	mte_check_tfsr_el1();
>  }

We need an isb() before mte_check_tfsr_el1() here as well, we only have
a dsb() in __switch_to(). We do have an isb() in update_sctlr_el1_tcf0()
but only if the check passed. Now, it's worth benchmarking how expensive
update_sctlr_el1_tcf0() is (i.e. an SCTLR_EL1 access + isb with
something like hackbench) and we could probably remove the check
altogether. In the meantime, you can add an isb() on the "else" path of
the above check.

-- 
Catalin