Re: [edk2-devel] [PATCH v8 07/10] OvmfPkg/SmmCpuFeaturesLib: call CPU hot-eject handler

["Laszlo Ersek" <lersek@redhat.com>]

On 02/23/21 08:45, Paolo Bonzini wrote:
> On 22/02/21 15:53, Laszlo Ersek wrote:
>>> +
>>> +  if (mCpuHotEjectData != NULL) {
>>> +    CPU_HOT_EJECT_HANDLER Handler;
>>> +
>>> +    Handler = mCpuHotEjectData->Handler;
>> This patch looks otherwise OK to me, but:
>>
>> In patch v8 08/10, we have a ReleaseMemoryFence(). (For now, it is only
>> expressed as a MemoryFence() call; we'll make that more precise later.)
>>
>> (1) I think that should be paired with an AcquireMemoryFence() call,
>> just before loading "mCpuHotEjectData->Handler" above -- for now, also
>> expressed as a MemoryFence() call only.
> 
> In Linux terms, there is a control dependency here.  However, it should
> at least be a separate statement to load mCpuHotEjectData (which from my
> EDK2 reminiscences should be a global) into a local variable.  So
> 
>   EjectData = mCPUHotEjectData;
>   // Optional AcquireMemoryFence here
>   if (EjectData != NULL) {
>     CPU_HOT_EJECT_HANDLER Handler;
> 
>     Handler = EjectData->Handler;
>     if (Handler != NULL) {
>       Handler (CpuIndex);
>     }
>   }

Yes, "mCPUHotEjectData" is a global.

"mCpuHotEjectData" itself is set up on the BSP (from the entry point
function of the PiSmmCpuSmmDxe driver), before any other APs have a
chance to execute any SMM-related code at all. Furthermore, once set up,
mCpuHotEjectData never changes -- it remains set to a particular
non-NULL value forever, or it remains NULL forever. (The latter case
applies when the possible CPU count is 1; IOW, then there is no AP at all.)

Because of that, I thought that the first comparison (mCpuHotEjectData
!= NULL) would not need any fence -- I thought it was similar to a
userspace program that (a) set a global variable in the "main" thread,
before calling pthread_create(), (b) treated the global variable as a
constant, ever after (meaning all threads).

However, mCpuHotEjectData->Handler is changed regularly (modified by the
BSP, and read "later" by all processors). That's why I thought the
acquire fence was needed in the following location:

  if (mCpuHotEjectData != NULL) {
    CPU_HOT_EJECT_HANDLER Handler;

    //
    // HERE -- AcquireMemoryFence()
    //
    Handler = mCpuHotEjectData->Handler;
    if (Handler != NULL) {
      Handler (CpuIndex);
    }
  }

Thanks!
Laszlo