CpuPause() might allow the CPU to go into a lower power state
state while we spin.

On X86, CpuPause() executes a PAUSE instruction which the Intel
and AMD specs describe as follows:

Intel:
  "PAUSE: 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."

AMD:
  "PAUSE: 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."

On RISC-V and ARM64, CpuPause() executes a NOP, which is no worse than
the tight loop we have.

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>
Reviewed-by: Laszlo Ersek <lersek@redhat.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



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Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>

Thanks
Liming
> -----邮件原件-----
> 发件人: Ankur Arora <ankur.a.arora@oracle.com>
> 发送时间: 2021年3月20日 2:14
> 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>; Laszlo Ersek <lersek@redhat.com>
> 主题: [RESEND v2] MdePkg: use CpuPause() in CpuDeadLoop()
> 
> CpuPause() might allow the CPU to go into a lower power state
> state while we spin.
> 
> On X86, CpuPause() executes a PAUSE instruction which the Intel
> and AMD specs describe as follows:
> 
> Intel:
>   "PAUSE: 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."
> 
> AMD:
>   "PAUSE: 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."
> 
> On RISC-V and ARM64, CpuPause() executes a NOP, which is no worse than
> the tight loop we have.
> 
> 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>
> Reviewed-by: Laszlo Ersek <lersek@redhat.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





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