Re: Linux loader EFI handover

[Andrei Borzenkov]

10.03.2016 17:21, Matt Fleming пишет:
> 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?
> 

Yes.

>>> 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.
> 

OK, even more reason to support EFI handover as part of normal linux loader.