Re: [PATCH 4/4] ArmPkg/DefaultExceptionHandlerLib: use console if available

[Laszlo Ersek <lersek@redhat.com>]

On 12/20/18 18:31, Ard Biesheuvel wrote:
> Print the minimal 'exception occurred' message to the console instead
> of straight to the serial port if the console is available. This makes
> such messages visible on systems where the console is graphical and
> the serial is not connected.
> 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c | 16 +++++++++++++---
>  ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c     |  7 ++++++-
>  ArmPkg/Library/DefaultExceptionHandlerLib/DefaultExceptionHandlerLib.inf    |  1 +
>  3 files changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
> index 1024bf48c63d..1aaf3c88f21e 100644
> --- a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
> +++ b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
> @@ -22,6 +22,7 @@
>  #include <Library/PrintLib.h>
>  #include <Library/ArmDisassemblerLib.h>
>  #include <Library/SerialPortLib.h>
> +#include <Library/UefiBootServicesTableLib.h>
>  
>  #include <Guid/DebugImageInfoTable.h>
>  #include <Protocol/DebugSupport.h>
> @@ -159,14 +160,23 @@ DefaultExceptionHandler (
>    INT32  Offset;
>  
>    if (mRecursiveException) {
> -    CharCount = AsciiSPrint (Buffer, sizeof (Buffer),"\nRecursive exception occurred while dumping the CPU state\n");
> -    SerialPortWrite ((UINT8 *) Buffer, CharCount);
> +    STATIC CHAR8 CONST Message[] = "\nRecursive exception occurred while dumping the CPU state\n";
> +
> +    if (gST->ConOut != NULL) {
> +      AsciiPrint (Message);
> +    } else {
> +      SerialPortWrite ((UINT8 *)Message, AsciiStrLen (Message));
> +    }
>      CpuDeadLoop ();
>    }
>    mRecursiveException = TRUE;
>  
>    CharCount = AsciiSPrint (Buffer,sizeof (Buffer),"\n\n%a Exception at 0x%016lx\n", gExceptionTypeString[ExceptionType], SystemContext.SystemContextAArch64->ELR);
> -  SerialPortWrite ((UINT8 *) Buffer, CharCount);
> +  if (gST->ConOut != NULL) {
> +    AsciiPrint (Buffer);
> +  } else {
> +    SerialPortWrite ((UINT8 *)Buffer, CharCount);
> +  }
>  
>    DEBUG_CODE_BEGIN ();
>      CHAR8  *Pdb, *PrevPdb;
> diff --git a/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c b/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
> index 0b9da031b47d..9159b579da6f 100644
> --- a/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
> +++ b/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
> @@ -21,6 +21,7 @@
>  #include <Library/PrintLib.h>
>  #include <Library/ArmDisassemblerLib.h>
>  #include <Library/SerialPortLib.h>
> +#include <Library/UefiBootServicesTableLib.h>
>  
>  #include <Guid/DebugImageInfoTable.h>
>  
> @@ -194,7 +195,11 @@ DefaultExceptionHandler (
>  
>    CharCount = AsciiSPrint (Buffer,sizeof (Buffer),"\n%a Exception PC at 0x%08x  CPSR 0x%08x ",
>           gExceptionTypeString[ExceptionType], SystemContext.SystemContextArm->PC, SystemContext.SystemContextArm->CPSR);
> -  SerialPortWrite ((UINT8 *) Buffer, CharCount);
> +  if (gST->ConOut != NULL) {
> +    AsciiPrint (Buffer);
> +  } else {
> +    SerialPortWrite ((UINT8 *)Buffer, CharCount);
> +  }
>  
>    DEBUG_CODE_BEGIN ();
>      CHAR8   *Pdb;
> diff --git a/ArmPkg/Library/DefaultExceptionHandlerLib/DefaultExceptionHandlerLib.inf b/ArmPkg/Library/DefaultExceptionHandlerLib/DefaultExceptionHandlerLib.inf
> index 7609f82d89a1..6bc48714c9dc 100644
> --- a/ArmPkg/Library/DefaultExceptionHandlerLib/DefaultExceptionHandlerLib.inf
> +++ b/ArmPkg/Library/DefaultExceptionHandlerLib/DefaultExceptionHandlerLib.inf
> @@ -42,6 +42,7 @@
>    PeCoffGetEntryPointLib
>    ArmDisassemblerLib
>    SerialPortLib
> +  UefiBootServicesTableLib
>  
>  [Guids]
>    gEfiDebugImageInfoTableGuid
> 

I feel that invoking such high level functionality from an exception
handler is risky, but I do understand this is a last resort / best
effort action, and if it *happens* to work on systems with no serial,
then it's already a win.

However, I'd suggest improving the robustness by preserving the serial
write first, and then performing the (optional) console write in
addition, second. I guess it can lead to the message appearing twice on
serial (if the console is available, and it happens to be multiplexed to
the serial port as well), but I think that's a better compromise than
possibly losing the message altogether.

I don't feel strongly about it either way, I just thought this worth
raising.

Thanks,
Laszlo