Re: Linux loader EFI handover (was: Bugs and tasks for 2.02[~rc1])

[Matt Fleming]

On Wed, 09 Mar, at 11:15:16PM, Andrei Borzenkov wrote:
> 09.03.2016 18:18, Matt Fleming пишет:
> > On Tue, 08 Mar, at 07:57:35AM, Andrei Borzenkov wrote:
> >>>> - 64-bit kernel on 32-bit platform like Baytrail can't work
> >>
> >> Do you mean "32 bit EFI"? If yes, why is it a problem?
> > 
> > The biggest issue is that there's no way right now for a boot loader
> > to tell the kernel that it needs to use a translation layer when
> > calling EFI services (we refer to this as the "thunk" layer in the
> > kernel) without going via the EFI handover protocol.
> > 
> > Obviously this could be achieved by writing the required code for GRUB
> > but it would be largely duplicated from the existing code EFI boot
> > stub code in the kernel. I don't think it's worth the effort.
> > 
> 
> That sounds like this should be supported irrespectively of secure boot
> then.
 
I'm not sure what you mean here. Are you suggesting to add support for
the EFI handover protocol to the regular linux loader?

> > The kernel figures out when to use the thunk layer by taking note of
> > which EFI handover offset entry point the boot loader entered from, we
> > include both a 32-bit and 64-bit entry point when CONFIG_EFI_MIXED is
> > enabled.
> > 
> 
> OK, looking at linuxefi patch, the only real difference from normal
> linux loader is that it restricts memory allocations to below 1G. Is it
> kernel requirement?
 
No, I'm not aware of such a requirement for modern kernels, though
it's possible there was a historical restriction.

> What to do if kernel is compiled without CONFIG_EFI_MIXED support?
> Should we fall back to traditional handover without calling into EFI
> stub or fail load completely?

Falling back to the traditional handover and disabling EFI runtime
services would be the best way to go. You can see whether mixed mode
is enabled by inspecting the ->xloadflags in the bzImage header.