Re: Clarification about InitializeCpuExceptionHandlers() and TGE bit in hcr_el2

[Andrew Fish <afish@apple.com>]

> On Sep 28, 2017, at 4:23 AM, Vabhav Sharma <vabhav.sharma@nxp.com> wrote:
> 
> Hi All,
> 
> I see that InitializeCpuExceptionHandlers() is called from DxeMain to take over exception handlers and later from ArmCpuDxe.
> Is there any specific purpose to call it from two places during dxe phase?
> 

Vabhav,

DxeMain is the DXE Core and that is like (micro) kernel and it is platform agnostic code. InitializeCpuExceptionHandlers() exists in that location to catch unhandled exceptions, especially in the case when no debugger stub is linked in. The CPU Dxe driver abstracts CPU specifics from the DXE Core and it adds supports for interrupts, cachability, etc. and the DXE Core uses services from this driver to abstract CPU implementation.

To make things even more complex on some platforms PEI and DXE run in entirely different modes. For example on x86 is is common for PEI to be 32-bit and and DXE to be 64-bit. This is mostly due to how complex it is to turn on memory, and the fact that there is no good place to put the page tables prior to memory init. 

I'll let the ARM folks comment on hcr_el2 usage. 

Thanks,

Andrew Fish 

> Additionally we are setting TGE bit three times in hcr_el2 during PrePei phase(ArmPlatformPkg/PrePi/AArch64/ArchPrePi.c)
> and Twice in Dxe phase: dxemain(),ArmCpuDxe
> 
> Please help to clarify or required to be fixed?
> 
> Regards,
> Vabhav
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