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

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Date:   Fri, 22 Oct 2021 09:51:40 +0800
From:   Xiongfeng Wang <wangxiongfeng2@...wei.com>
To:     Catalin Marinas <catalin.marinas@....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

Hi Catalin,

On 2021/10/16 1:59, Catalin Marinas wrote:
> 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.

In the last mail, I was suggested to investigate the code in other architecture
to find if there exists similar interface. I searched 'merg' in the code and
didn't find similar interface.

The only thing similar I found is in Intel Software Developer's Manual. It says
"Write Combining (WC) ... Writes may be delayed and combined in the write
combining buffer (WC buffer) to reduce memory accesses. If the WC buffer is
partially filled, the writes may be delayed until the next occurrence of a
serializing event; such as an SFENCE or MFENCE instruction, CPUID or other
serializing instruction, a read or write to uncached memory, an interrupt
occurrence, or an execution of a LOCK instruction (including one with an
XACQUIRE or XRELEASE prefix)."
Maybe this is more like the write combine buffer flushing, not prevent merging.
Sorry I still didn't understand the difference clearly.

How about a common name called 'merge_prohibit_hint()'? Could you give me some
suggestions ?

Thanks,
Xiongfeng

>