Re: [edk2-devel] [edk2] AP core : call GetPeiServicesTablePointer function hang

["Michael D Kinney" <michael.d.kinney@intel.com>]

PEI Phase execution environment is focused on BSP execution.
The PEI Services and most of the PPI services are not designed
to be MP safe.  Since it is not safe for an AP to call PEI Services,
the IDT entry with the pointer to the PEI Service Table is
not available to an AP to prevent code from incorrectly calling
a PEI Service from an AP.

It is legal to run code on an AP in the PEI Phase, but the
code executed must be carefully designed to not use PEI Services
and is usually designed to execute using only stack/memory and 
system resources that have assigned for exclusive access by an AP.
This is usually communicated from BSP to an AP through a mailbox
in memory.  If an AP needs something like a PEI Service (e.g. 
memory allocation or logging), then the AP should make that 
request to the BSP to perform that operation for the AP.

Mike

> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On
> Behalf Of Tiger Liu(BJ-RD)
> Sent: Monday, November 18, 2019 1:08 AM
> To: devel@edk2.groups.io
> Subject: [edk2-devel] [edk2] AP core : call
> GetPeiServicesTablePointer function hang
> 
> Dear All:
> I met a question during debug multi-processor code.
> 
> I found if AP's procedure call
> GetPeiServicesTablePointer function , then it would
> hang.
> GetPeiServicesTablePointer (
>   VOID
>   )
> {
>   CONST EFI_PEI_SERVICES  **PeiServices;
>   IA32_DESCRIPTOR   Idtr;
> 
>   AsmReadIdtr (&Idtr);
>   PeiServices = (CONST EFI_PEI_SERVICES **)
> (*(UINTN*)(Idtr.Base - sizeof (UINTN)));
>   ASSERT (PeiServices != NULL);
>   return PeiServices;
> }
> 
> It seems BSP and AP use different IDT table? Why?
> 
> Thanks
> 
> 
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