Re: [PATCH v2 1/4] UefiCpuPkg/ResetVector/Vtf0: document segment register setup

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 15 November 2017 at 21:57, Laszlo Ersek <lersek@redhat.com> wrote:
> "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 <ard.biesheuvel@linaro.org>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

> ---
>
> Notes:
>     v2:
>     - new patch [Jordan]
>
>  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 146df600a63b..bc68c8dd749a 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 ebfb9015d49c..57f080688b6f 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
>  ;
> --
> 2.14.1.3.gb7cf6e02401b
>
>