Re: 回复: [PATCH] MdePkg: use CpuPause() in CpuDeadLoop()

["Ankur Arora" <ankur.a.arora@oracle.com>]

On 2021-03-16 6:08 p.m., gaoliming wrote:
> Ankur:
>    Can you give the detail usage for the lower power state when enter into
> CpuDeadLoop()?

I'm not really familiar with ARM and RiscV, both of which also have implementations
for CpuPause(), for the details for X86 variants are below.

 From Intel's instruction-set reference:
"An additional function of the PAUSE instruction is to reduce the power consumed
by a processor while executing a spin loop. A processor can execute a spin-wait
loop extremely quickly, causing the processor to consume a lot of power while it
waits for the resource it is spinning on to become available. Inserting a pause
instruction in a spin-wait loop greatly reduces the processor’s power
consumption."

And, from AMD's instruction-set reference:
"Improves the performance of spin loops, by providing a hint to the processor
that the current code is in a spin loop. The processor may use this to optimize
power consumption while in the spin loop. Architecturally, this instruction
behaves like a NOP instruction."

Happy to add this to the commit message if you think it'll be useful.

Thanks
Ankur

> 
> Thanks
> Liming
>> -----邮件原件-----
>> 发件人: Ankur Arora <ankur.a.arora@oracle.com>
>> 发送时间: 2021年3月17日 6:59
>> 收件人: devel@edk2.groups.io
>> 抄送: Ankur Arora <ankur.a.arora@oracle.com>; Liming Gao
>> <gaoliming@byosoft.com.cn>; Michael D Kinney
>> <michael.d.kinney@intel.com>
>> 主题: [PATCH] MdePkg: use CpuPause() in CpuDeadLoop()
>>
>> Use CpuPause() to allow the CPU to go into a lower power state
>> state while we spin wait.
>>
>> Cc: Liming Gao <gaoliming@byosoft.com.cn>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
>> ---
>>   MdePkg/Library/BaseLib/CpuDeadLoop.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/MdePkg/Library/BaseLib/CpuDeadLoop.c
>> b/MdePkg/Library/BaseLib/CpuDeadLoop.c
>> index 9e110cacbc96..3cd304351a65 100644
>> --- a/MdePkg/Library/BaseLib/CpuDeadLoop.c
>> +++ b/MdePkg/Library/BaseLib/CpuDeadLoop.c
>> @@ -28,5 +28,7 @@ CpuDeadLoop (
>>   {
>>     volatile UINTN  Index;
>>
>> -  for (Index = 0; Index == 0;);
>> +  for (Index = 0; Index == 0;) {
>> +    CpuPause();
>> +  }
>>   }
>> --
>> 2.9.3
> 
> 
>