Re: [PATCH net-next 1/3] arm64: barrier: add DGH macros to control memory accesses merging

[Xiongfeng Wang <wangxiongfeng2@huawei.com>]

Hi,

On 2021/6/29 19:11, Xiongfeng Wang wrote:
> Hi Will,
> 
> On 2021/6/22 20:16, Will Deacon wrote:
>> On Tue, Jun 22, 2021 at 07:11:09PM +0800, Guangbin Huang wrote:
>>> From: Xiongfeng Wang <wangxiongfeng2@huawei.com>
>>>
>>> DGH prohibits merging memory accesses with Normal-NC or Device-GRE
>>> attributes before the hint instruction with any memory accesses
>>> appearing after the hint instruction. Provide macros to expose it to the
>>> arch code.
>>
>> Hmm.
>>
>> The architecture states:
>>
>>   | DGH is a hint instruction. A DGH instruction is not expected to be
>>   | performance optimal to merge memory accesses with Normal Non-cacheable
>>   | or Device-GRE attributes appearing in program order before the hint
>>   | instruction with any memory accesses appearing after the hint instruction
>>   | into a single memory transaction on an interconnect.
>>
>> which doesn't make a whole lot of sense to me, in all honesty.
>>
>>> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
>>> Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
>>> Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
>>> ---
>>>  arch/arm64/include/asm/assembler.h | 7 +++++++
>>>  arch/arm64/include/asm/barrier.h   | 1 +
>>>  2 files changed, 8 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
>>> index 8418c1bd8f04..d723899328bd 100644
>>> --- a/arch/arm64/include/asm/assembler.h
>>> +++ b/arch/arm64/include/asm/assembler.h
>>> @@ -90,6 +90,13 @@
>>>  	.endm
>>>  
>>>  /*
>>> + * Data gathering hint
>>> + */
>>> +	.macro	dgh
>>> +	hint	#6
>>> +	.endm
>>> +
>>> +/*
>>>   * RAS Error Synchronization barrier
>>>   */
>>>  	.macro  esb
>>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>>> index 451e11e5fd23..02e1735706d2 100644
>>> --- a/arch/arm64/include/asm/barrier.h
>>> +++ b/arch/arm64/include/asm/barrier.h
>>> @@ -22,6 +22,7 @@
>>>  #define dmb(opt)	asm volatile("dmb " #opt : : : "memory")
>>>  #define dsb(opt)	asm volatile("dsb " #opt : : : "memory")
>>>  
>>> +#define dgh()		asm volatile("hint #6" : : : "memory")
>>
>> Although I'm fine with this in arm64, I don't think this is the interface
>> which drivers should be using. Instead, once we know what this instruction
>> is supposed to do, we should look at exposing it as part of the I/O barriers
>> and providing a NOP implementation for other architectures. That way,
>> drivers can use it without having to have the #ifdef CONFIG_ARM64 stuff that
>> you have in the later patches here.
> 
> How about we adding a interface called flush_wc_writeX(), which can be used to
> flush the write-combined buffers to the device immediately.
> I found it has been disscussed in the below link, but it is unnessary in their
> situation.
> https://patchwork.ozlabs.org/project/netdev/patch/20200102180830.66676-3-liran.alon@oracle.com/

Do you have some suggestions on this problem ? How about we adding an interface
called flush_wc_writeX() ?

Thanks,
Xiongfeng

> 
> Thanks,
> Xiongfeng
> 
>>
>> Will
>>
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>>