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From: Will Deacon <will@kernel.org>
To: Pingfan Liu <kernelfans@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mark Rutland <mark.rutland@arm.com>,
	Andrei Vagin <avagin@gmail.com>, Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH 2/2] arm64/arch_timer: replace arch_counter_enforce_ordering() with isb
Date: Tue, 30 Mar 2021 12:05:53 +0100	[thread overview]
Message-ID: <20210330110552.GA5707@willie-the-truck> (raw)
In-Reply-To: <20210330105719.47760-3-kernelfans@gmail.com>

On Tue, Mar 30, 2021 at 06:57:19PM +0800, Pingfan Liu wrote:
> The description of getting counter value is not very clear. [1]
> 'mrs Xt, cntpct' may execute out of program order, either forward or
> backward.
> 
> Now taking a look at this group of getting counter routines. All of them
> are called from sched_clock(). And there is an isb to protect forward
> speculation. But there is no isb for the backward speculation.
> 
> The current code enforces read dependency instructions anchored on
> getting counter. But it is not enough to protect against other no
> dependency instructions, and even function call can not prevent the
> speculation between getting counter and them.

Which "no dependency instructions"?

> 
> Replacing arch_counter_enforce_ordering() with isb to achieve the aim.
> 
> [1]: AArch64 Programmer's Guides Generic Timer:  3.1. Count and frequency

I wouldn't trust that guide as far as I can throw it.

Please describe the problem you're trying to solve, and hopefully I can
help. ISB is an expensive instruction so we need a good justification to
add it here (i.e. an example of why the current scheme is not correct).

Thanks,

Will

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  reply	other threads:[~2021-03-30 11:08 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-30 10:57 [PATCH 0/2] arm64/timer: trival fix for sync inside counter Pingfan Liu
2021-03-30 10:57 ` [PATCH 1/2] arm64/gettimeofday: correct the note about isb in __arch_get_hw_counter() Pingfan Liu
2021-03-30 11:08   ` Will Deacon
2021-03-30 10:57 ` [PATCH 2/2] arm64/arch_timer: replace arch_counter_enforce_ordering() with isb Pingfan Liu
2021-03-30 11:05   ` Will Deacon [this message]
2021-03-31  9:20     ` Pingfan Liu

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