Good catch!

Reviewed-by: Nate DeSimone <nathaniel.l.desim...@intel.com>

-----Original Message-----
From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Ni, Ray
Sent: Tuesday, October 31, 2023 1:22 AM
To: devel@edk2.groups.io
Cc: Chiu, Chasel <chasel.c...@intel.com>
Subject: [edk2-devel] [PATCH] IntelFsp2Pkg/SwitchStack: Reserve 32B when 
calling C function in 64bit

When FSP runs in API mode, it saves the IDTR in its own stack then switches to 
bootloader's stack before it returns from FspMemoryInit.
Next time when the bootloader calls TempRamExit, FSP switches to its own stack 
and restores IDTR from its stack saved earlier.

However, due to a bug in BaseFspSwitchStackLib, the IDTR saved on FSP's stack 
might be corrupted that results the following TempRamExit call fails inside FSP 
due to PeiServices pointer cannot be retrieved from IDT.base - 8.

The bug is the assembly code doesn't reserve 32 bytes before calling the C 
routine in 64bit. According to the x86-64 calling convention, caller is 
responsible for allocating 32 bytes of "shadow space" on the stack right before 
calling the function (regardless of the actual number of parameters used).

When FSP is built in optimization-off mode, the C routine makes use of the 
32-byte "shadow space" which is not reserved by the assembly caller. That 
causes the IDTR saved on the stack is corrupted by the C routine.
The patch fixes so by reserving the 32 bytes before calling C routine.

Signed-off-by: Ray Ni <ray...@intel.com>
Cc: Chasel Chiu <chasel.c...@intel.com>
M: Nate DeSimone <nathaniel.l.desim...@intel.com>
M: Duggapu Chinni B <chinni.b.dugg...@intel.com>
M: Ray Han Lim Ng <ray.han.lim...@intel.com>
R: Star Zeng <star.z...@intel.com>
R: Ted Kuo <ted....@intel.com>
R: Ashraf Ali S <ashraf.al...@intel.com>
R: Susovan Mohapatra <susovan.mohapa...@intel.com>
---
 IntelFsp2Pkg/Library/BaseFspSwitchStackLib/X64/Stack.nasm | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/IntelFsp2Pkg/Library/BaseFspSwitchStackLib/X64/Stack.nasm 
b/IntelFsp2Pkg/Library/BaseFspSwitchStackLib/X64/Stack.nasm
index 1ea1220608..e3a7cf002f 100644
--- a/IntelFsp2Pkg/Library/BaseFspSwitchStackLib/X64/Stack.nasm
+++ b/IntelFsp2Pkg/Library/BaseFspSwitchStackLib/X64/Stack.nasm
@@ -1,6 +1,6 @@
 
;------------------------------------------------------------------------------ 
;-; Copyright (c) 2022, Intel Corporation. All rights reserved.<BR>+; Copyright 
(c) 2022 - 2023, Intel Corporation. All rights reserved.<BR> ; 
SPDX-License-Identifier: BSD-2-Clause-Patent ; ; Abstract:@@ -60,7 +60,9 @@ 
ASM_PFX(FspSwitchStack):
      ; Load new stack     mov     rcx, rsp+    sub     rsp, 0x20     call    
ASM_PFX(SwapStack)+    add     rsp, 0x20     mov     rsp, rax      ; Restore 
previous contexts-- 
2.39.1.windows.1



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