From 9f75aacc7ab5410345b4811901a78a375851aa8b Mon Sep 17 00:00:00 2001 From: Laszlo Ersek Date: Wed, 15 Nov 2017 19:00:17 +0100 Subject: [PATCH] UefiCpuPkg/ResetVector/Vtf0: document segment register setup "Main.asm" calls TransitionFromReal16To32BitFlat (and does some other things) before it jumps to the platform's SEC entry point. TransitionFromReal16To32BitFlat enters big real mode, and sets the DS, ES, FS, GS, and SS registers to offset ("selector") LINEAR_SEL in the GDT (defined in "UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm"). The GDT entry ("segment descriptor") at LINEAR_SEL defines a segment covering the full 32-bit address space, meant for "read/write data". Document this fact for all the affected segment registers, as output parameters for TransitionFromReal16To32BitFlat, saying "Selector allowing flat access to all addresses". For 64-bit SEC, "Main.asm" calls Transition32FlatTo64Flat in addition, between calling TransitionFromReal16To32BitFlat and jumping to the SEC entry point. Transition32FlatTo64Flat enters long mode. In long mode, segmentation is largely ignored: - all segments are considered flat (covering the whole 64-bit address space), - with the (possible) exception of FS and GS, whose bases can still be changed, albeit with new methods, not through the GDT. (Through the IA32_FS_BASE and IA32_GS_BASE Model Specific Registers, and/or the WRFSBASE, WRGSBASE and SWAPGS instructions.) Thus, document the segment registers with the same "Selector allowing flat access to all addresses" language on the "Main.asm" level too, since that is valid for both 32-bit and 64-bit modes. (Technically, "Main.asm" does not return, but RBP/EBP, passed similarly to the SEC entry point, is already documented as an output parameter.) Cc: Ard Biesheuvel Cc: Eric Dong Cc: Jordan Justen Suggested-by: Jordan Justen Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Laszlo Ersek Acked-by: Ard Biesheuvel Reviewed-by: Jordan Justen --- UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm | 6 ++++++ UefiCpuPkg/ResetVector/Vtf0/Main.asm | 5 +++++ 2 files changed, 11 insertions(+) diff --git a/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm b/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm index 146df600a6..bc68c8dd74 100644 --- a/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm +++ b/UefiCpuPkg/ResetVector/Vtf0/Ia16/Real16ToFlat32.asm @@ -21,6 +21,12 @@ BITS 16 ; ; Modified: EAX, EBX ; +; @param[out] DS Selector allowing flat access to all addresses +; @param[out] ES Selector allowing flat access to all addresses +; @param[out] FS Selector allowing flat access to all addresses +; @param[out] GS Selector allowing flat access to all addresses +; @param[out] SS Selector allowing flat access to all addresses +; TransitionFromReal16To32BitFlat: debugShowPostCode POSTCODE_16BIT_MODE diff --git a/UefiCpuPkg/ResetVector/Vtf0/Main.asm b/UefiCpuPkg/ResetVector/Vtf0/Main.asm index ebfb9015d4..57f080688b 100644 --- a/UefiCpuPkg/ResetVector/Vtf0/Main.asm +++ b/UefiCpuPkg/ResetVector/Vtf0/Main.asm @@ -24,6 +24,11 @@ BITS 16 ; @param[in,out] DI 'BP': boot-strap processor, or ; 'AP': application processor ; @param[out] RBP/EBP Address of Boot Firmware Volume (BFV) +; @param[out] DS Selector allowing flat access to all addresses +; @param[out] ES Selector allowing flat access to all addresses +; @param[out] FS Selector allowing flat access to all addresses +; @param[out] GS Selector allowing flat access to all addresses +; @param[out] SS Selector allowing flat access to all addresses ; ; @return None This routine jumps to SEC and does not return ;