Bottomley: Owning your Windows 8 UEFI Platform [LWN.net]

Bottomley: Owning your Windows 8 UEFI Platform

Posted Feb 25, 2013 16:50 UTC (Mon) by nix (subscriber, #2304)
In reply to: Bottomley: Owning your Windows 8 UEFI Platform by khim
Parent article: Bottomley: Owning your Windows 8 UEFI Platform

Uh... EFI-supporting BIOSes can also boot in BIOS mode for backward compatibility, and (obviously) still boot the Management Engine long before they make the decision to do that. (My current desktop is booted in just that way, as is the desktop of most Linux users using recent processors, and most existing Windows users on such machines.

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Bottomley: Owning your Windows 8 UEFI Platform

Posted Feb 25, 2013 20:33 UTC (Mon) by etienne (guest, #25256) [Link] (1 responses)

Are all EFI PC able to run the EFI stack, init the memory and processor, and reboot to BIOS if they detect the user wants to boot the BIOS way (for instance to run an El-Torito CDROM), or are they loading the EFI stack and try to simulate a real mode BIOS - and then good luck if you try to run memtester which would overwrite the EFI code.

Bottomley: Owning your Windows 8 UEFI Platform

Posted Feb 25, 2013 21:26 UTC (Mon) by khim (subscriber, #9252) [Link]

Are all EFI PC able to run the EFI stack, init the memory and processor, and reboot to BIOS if they detect the user wants to boot the BIOS way (for instance to run an El-Torito CDROM), or are they loading the EFI stack and try to simulate a real mode BIOS

Usually it's a combination: they start as EFI and then at some point switch to old real mode BIOS+SMM code mix.

And then good luck if you try to run memtester which would overwrite the EFI code.

YMMV: some will survive memtester (if they keep the important bits hidden away in SMM), some may crash — especially when USB is involved (the first example Google found).

Bottomley: Owning your Windows 8 UEFI Platform

Posted Feb 25, 2013 20:42 UTC (Mon) by khim (subscriber, #9252) [Link] (1 responses)

EFI-supporting BIOSes can also boot in BIOS mode for backward compatibility

"EFI-supporting BIOS" is an oxymoron. Most such BIOSes are, in fact, thin BIOS emulation shims on top of the real thing: EFI-based firmware.

and (obviously) still boot the Management Engine long before they make the decision to do that

Yup. They start as EFI and then pretend they are BIOS. This combines worst sides of EFI and BIOS approaches but makes older OSes happy if you are lucky. It still means that without EFI your hardware will not boot.

Bottomley: Owning your Windows 8 UEFI Platform

Posted Feb 26, 2013 15:38 UTC (Tue) by nix (subscriber, #2304) [Link]

That entirely depends on what you mean by 'without EFI'. From my POV, in BIOS mode, my motherboard's EFI is just an implementation detail used by very early boot, of no interest once software I can control is running: among other things, I don't need an EFI partition, an EFI bootloader, or any of the other EFI PE stuff. It might as well be an old-style BIOS from my perspective. (The sensors don't work, but that's got nothing to do with EFI and everything to do with a sensor vendor and motherboard vendor who refuse to release any information on the sensor chip: each refers you to the other for info and neither will give you any).