Syslinux - Oct 2013 - Booting legacy Linux kernel in EFI mode

Hello,

My question is quite straightforward, still I couldn't find an anwser
on the interweb.
Is it possible to boot the Linux kernels I used to boot in legacy
mode, but now in EFI mode?

I mean, I would like to keep all my PXE settings with my menus and
kernels and stuff, but now boot them in PXE-EFI mode.

I've been able to boot on syslinux.efi and keep my config files, but
when I try to boot on a linux kernel, I get an error like "no such
file or directory" while I see it download the file from the tftp.

Is this a bug in syslinux?
Or is there a deep incompatibility?
And (a bit out of scope) if I get my old kernel to boot in EFI mode,
will the driver efivars have access to the EFI variables? Or does the
kernel need to be recompiled specially for EFI?


Regards,

Celelibi

On Sun, Sep 22, 2013 at 3:09 PM, Celelibi <celelibi at gmail.com>
wrote:
> Hello,
>
> My question is quite straightforward, still I couldn't find an anwser
> on the interweb.
> Is it possible to boot the Linux kernels I used to boot in legacy
> mode, but now in EFI mode?
I believe unless the kernel specifically supports EFI boot, it can not
be booted.
> I mean, I would like to keep all my PXE settings with my menus and
> kernels and stuff, but now boot them in PXE-EFI mode.
>
> I've been able to boot on syslinux.efi and keep my config files, but
> when I try to boot on a linux kernel, I get an error like "no such
> file or directory" while I see it download the file from the tftp.
>
> Is this a bug in syslinux?
> Or is there a deep incompatibility?
> And (a bit out of scope) if I get my old kernel to boot in EFI mode,
> will the driver efivars have access to the EFI variables? Or does the
> kernel need to be recompiled specially for EFI?
Doubtful.  Yes (although I think most EFI kernels I've seen will
function for both).

--Gene

On Tue, Sep 24, 2013 at 11:57 PM, Gene Cumm <gene.cumm at gmail.com>
wrote:
> On Sun, Sep 22, 2013 at 3:09 PM, Celelibi <celelibi at gmail.com>
wrote:
>> And (a bit out of scope) if I get my old kernel to boot in EFI mode,
>> will the driver efivars have access to the EFI variables? Or does the
>> kernel need to be recompiled specially for EFI?
>
> Doubtful.  Yes (although I think most EFI kernels I've seen will
> function for both).

    Just to be clear, it requires enabling of `CONFIG_EFI_STUB`
configuration variable right?

    Because although I'm using one of the pre-release versions of
`6.02` which should support x32 kernel loading (with x64 UEFI) it
still doesn't work for me.  (Although I haven't fiddle too much with
it.)

    Ciprian.

2013/9/24, Gene Cumm <gene.cumm at gmail.com>:
> On Sun, Sep 22, 2013 at 3:09 PM, Celelibi <celelibi at gmail.com>
wrote:
>> Hello,
>>
>> My question is quite straightforward, still I couldn't find an
anwser
>> on the interweb.
>> Is it possible to boot the Linux kernels I used to boot in legacy
>> mode, but now in EFI mode?
>
> I believe unless the kernel specifically supports EFI boot, it can not
> be booted.
>
>> I mean, I would like to keep all my PXE settings with my menus and
>> kernels and stuff, but now boot them in PXE-EFI mode.
>>
>> I've been able to boot on syslinux.efi and keep my config files,
but
>> when I try to boot on a linux kernel, I get an error like "no such
>> file or directory" while I see it download the file from the tftp.
>>
>> Is this a bug in syslinux?
>> Or is there a deep incompatibility?
>> And (a bit out of scope) if I get my old kernel to boot in EFI mode,
>> will the driver efivars have access to the EFI variables? Or does the
>> kernel need to be recompiled specially for EFI?
>
> Doubtful.  Yes (although I think most EFI kernels I've seen will
> function for both).
>
> --Gene
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>

Sorry for answering that late. (I had some issues with my server's
Ethernet NIC, and without it, no PXE / syslinux.)

So, it's time to be a bit more specific.
The kernel I want to boot in EFI mode is a debian installer. It *has*
a module "efivars" loaded. Which means this kernel has been compiled
with CONFIG_EFI_VARS, which itself depends on CONFIG_EFI. Therefore I
should be able to boot this kernel in EFI mode.

I don't remember whether I tried or not, so I'll re-test ASAP.
However, as I discussed on IRC, the 'file' command only say "x86
boot sector".


Celelibi