linux-kernel - Re: [RFC PATCH v2] arm64: barrier: add macro dgh() to control memory accesses merging

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Message-ID: <YWnBngJeIvV2S5IB@arm.com>
Date:   Fri, 15 Oct 2021 18:59:58 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Xiongfeng Wang <wangxiongfeng2@...wei.com>
Cc:     mark.rutland@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        moyufeng@...wei.com
Subject: Re: [RFC PATCH v2] arm64: barrier: add macro dgh() to control memory
 accesses merging

On Fri, Oct 15, 2021 at 05:05:11PM +0800, Xiongfeng Wang wrote:
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 451e11e5fd23..d71a7457d619 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -18,6 +18,14 @@
>  #define wfe()		asm volatile("wfe" : : : "memory")
>  #define wfi()		asm volatile("wfi" : : : "memory")
>  
> +/*
> + * Data Gathering Hint:
> + * This instruction prohibits merging memory accesses with Normal-NC or
> + * Device-GRE attributes before the hint instruction with any memory accesses
> + * appearing after the hint instruction.
> + */
> +#define dgh()		asm volatile("hint #6" : : : "memory")

On its own, this patch doesn't do anything. It's more interesting to see
how it will be used and maybe come up with a common name that other
architectures would share (or just implement as no-opp). I'm not sure
there was any conclusion last time we discussed this.

-- 
Catalin