Re: [PATCH 3/5] GenGv: Arm: support images entered in Thumb mode

["Sami Mujawar" <sami.mujawar@arm.com>]

Hi Etienne,

Thank you for this patch.

Please find my comments below marked [SAMI].

Regards,

Sami Mujawar
On 04/05/2021 04:20 PM, Etienne Carriere wrote:
> Change GenFv for Arm architecture to generate a specific jump
> instruction as image entry instruction, when the target entry label
> is assembled with Thumb instruction set. This is possible since
> SecCoreEntryAddress value fetched from the PE32 as its LSBit set when
> the entry instruction executes in Thumb mode.
>
> Cc: Bob Feng <bob.c.feng@intel.com>
> Cc: Liming Gao <gaoliming@byosoft.com.cn>
> Cc: Achin Gupta <achin.gupta@arm.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Leif Lindholm <leif@nuviainc.com>
> Cc: Sughosh Ganu <sughosh.ganu@linaro.org>
> Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
> ---
>   BaseTools/Source/C/GenFv/GenFvInternalLib.c | 38 +++++++++++++++-----
>   1 file changed, 29 insertions(+), 9 deletions(-)
>
> diff --git a/BaseTools/Source/C/GenFv/GenFvInternalLib.c b/BaseTools/Source/C/GenFv/GenFvInternalLib.c
> index 6e296b8ad6..3af65146f6 100644
> --- a/BaseTools/Source/C/GenFv/GenFvInternalLib.c
> +++ b/BaseTools/Source/C/GenFv/GenFvInternalLib.c
> @@ -34,9 +34,27 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
>   #include "FvLib.h"
>   #include "PeCoffLib.h"
>   
> -#define ARMT_UNCONDITIONAL_JUMP_INSTRUCTION       0xEB000000
>   #define ARM64_UNCONDITIONAL_JUMP_INSTRUCTION      0x14000000
>   
> +/*
> + * Arm instruction to jump to Fv enry instruction in Arm or Thumb mode.
> + * From ARM Arch Ref Manual versions b/c/d, section A8.8.25 BL, BLX (immediate)
> + * BLX (encoding A2) branches to offset in Thumb instruction set mode.
> + * BL (encoding A1) branches to offset in Arm instruction set mode.
> + */
> +#define ARM_JUMP_OFFSET_MAX        0xffffff
> +#define ARM_JUMP_TO_ARM(Offset)    (0xeb000000 | ((Offset - 8) >> 2))
> +
> +#define _ARM_JUMP_TO_THUMB(Imm32)  (0xfa000000 | \
> +                                    (((Imm32) & (1 << 1)) << (24 - 1)) | \
> +                                    (((Imm32) >> 2) & 0x7fffff))
> +#define ARM_JUMP_TO_THUMB(Offset)  _ARM_JUMP_TO_THUMB((Offset) - 8)
> +
> +/*
> + * Arm instruction to retrun from exception (MOVS PC, LR)
> + */
> +#define ARM_RETURN_FROM_EXCEPTION  0xE1B0F07E
> +
>   BOOLEAN mArm = FALSE;
>   BOOLEAN mRiscV = FALSE;
>   STATIC UINT32   MaxFfsAlignment = 0;
> @@ -2203,23 +2221,25 @@ Returns:
>       // if we found an SEC core entry point then generate a branch instruction
>       // to it and populate a debugger SWI entry as well
>       if (UpdateVectorSec) {
> +      UINT32                    EntryOffset;
>   
>         VerboseMsg("UpdateArmResetVectorIfNeeded updating ARM SEC vector");
>   
> -      // B SecEntryPoint - signed_immed_24 part +/-32MB offset
> -      // on ARM, the PC is always 8 ahead, so we're not really jumping from the base address, but from base address + 8
> -      ResetVector[0] = (INT32)(SecCoreEntryAddress - FvInfo->BaseAddress - 8) >> 2;
> +      EntryOffset = (INT32)(SecCoreEntryAddress - FvInfo->BaseAddress);
>   
> -      if (ResetVector[0] > 0x00FFFFFF) {
> -        Error(NULL, 0, 3000, "Invalid", "SEC Entry point must be within 32MB of the start of the FV");
> +      if (EntryOffset > ARM_JUMP_OFFSET_MAX) {
> +          Error(NULL, 0, 3000, "Invalid", "SEC Entry point offset above 1MB of the start of the FV");
>           return EFI_ABORTED;
>         }
>   
> -      // Add opcode for an unconditional branch with no link. i.e.: " B SecEntryPoint"
> -      ResetVector[0] |= ARMT_UNCONDITIONAL_JUMP_INSTRUCTION;
> +      if (SecCoreEntryAddress & 1) {
[SAMI] if condition should evaluate a Boolean value. See 
https://edk2-docs.gitbook.io/edk-ii-c-coding-standards-specification/5_source_files/57_c_programming#5-7-2-1-boolean-values-variable-type-boolean-do-not-require-explicit-comparisons-to-true-or-false
With that changed.
[/SAMI]
> +        ResetVector[0] = ARM_JUMP_TO_THUMB(EntryOffset);
> +      } else {
> +        ResetVector[0] = ARM_JUMP_TO_ARM(EntryOffset);
> +      }
>   
>         // SWI handler movs   pc,lr. Just in case a debugger uses SWI
> -      ResetVector[2] = 0xE1B0F07E;
> +      ResetVector[2] = ARM_RETURN_FROM_EXCEPTION;
>   
>         // Place holder to support a common interrupt handler from ROM.
>         // Currently not supported. For this to be used the reset vector would not be in this FV