Re: Help Required legacy booting from an EFI

[Laszlo Ersek <lersek@redhat.com>]

On 09/19/16 17:00, Saqib Khan wrote:
> Hello all, this is my first post to this list and i am very new to edk2
> development.
> 
> I started by building BootManagerMenuApp with following functionality
> 
>    - Boot from BootManagerManueApp
>    - Run an Helloworld EFI
>    - Run windows 7 EFI to boot windows 7
> 
> Now i want to develop an EFI which will first run Helloworld EFI then it
> will boot windows 7 which is installed on legacy boot (NOT EFI).
> 
> I have looked through http://bluestop.org/edk2/docs/trunk/ documentation
> thoroughly but i am not able to get any thing for legacy boot from an EFI.
> 
> 
> It will be helpful if someone guide me the flow for legacy booting from an
> EFI .

You can add your EFI application to a SysPrep#### load option, then the
legacy-booted Windows OS to a normal Boot#### load option.

See "3.1.7 Required System Preparation Applications" in the UEFI-2.6
spec, about the ordering between SysPrep#### and Boot####.

Furthermore, legacy boot can be triggered by using a BBS (BIOS Boot
Specification) device path in the Boot#### option. This devpath node
type is documented in UEFI-2.6 "Table 98. Device Node Table", and in the
Compatibility Support Module Specification, rev 0.98, "3.2.1 Legacy BIOS
Protocol", near BBS_BBS_DEVICE_PATH.

For legacy boot to work, your platform firmware must of course include a
Compatibility Support Module.

As an example platform, SeaBIOS can be built as a CSM, then embedded
into OVMF with the -D CSM_ENABLE build switch. (Grep
OvmfPkg/OvmfPkg*.{dsc,fdf} for CSM_ENABLE.)

Specifically for Windows 7 though, you don't need a full CSM in order to
boot it under UEFI. You can boot it in UEFI mode just fine, as long as
you have some VBE services installed in the C segment. Windows 7 will
run those functions in its internal real-mode emulator, for setting up
the display:

https://en.wikipedia.org/wiki/Virtual_DOS_machine

That is, the VBE functions will not be executed natively, they will be
interpreted / emulated.

We utilized this fact for booting UEFI Windows 7 without a CSM on QEMU,
on the virtual QXL video card; see OvmfPkg/QemuVideoDxe/VbeShim.*.

Thanks
Laszlo