Re: Grub2: add UEFI support for accessing memory address above 4GB.

["Vladimir 'phcoder' Serbinenko" <phcoder@gmail.com>]

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On Tue, Mar 7, 2017, 09:09 Michel Hermier <michel.hermier@gmail.com> wrote:

>
>
> Le 7 mars 2017 17:24, "Vladimir 'phcoder' Serbinenko" <phcoder@gmail.com>
> a écrit :
>
>
>
> On Tue, Mar 7, 2017, 08:15 Leif Lindholm <leif.lindholm@linaro.org> wrote:
>
> On Tue, Mar 07, 2017 at 01:55:01AM +0000, Yufuping wrote:
> > Who can add the new feature for grub2:
> > Add UEFI support for accessing memory address above 4GB.
>
> Presumably you mean for x86_64?
> Since GRUB supports all 5 architectures currently supported by the
> UEFI specification, 3 of which are 64-bit, it is useful to be a bit
> more precise.
>
> > When using grub2 as PXE downloading engine, grub2 can get initrd
> > file from network and put it to memory above 4GB.
>
> I'd like to know more about the usecase. Generally you should avoid
> downloading or loading too large files in bootloader. I.a. TFTP protocol
> has problems with files over about 100MIB. Generally you should download
> only kernel + initrd and rest of the system should be on iSCSI or NFS.
>
>
> I can think of nothing particularly related to PXE here.
> The x86_64 port currently sets GRUB_EFI_MAX_USABLE_ADDRESS to
> 0xffffffff or 0x7fffffff, depending on toolchain configuration.
>
> ARM64 sets it to 0xffffffffffffULL, and that works fine.
>
> I seem to recall that the x86_64 port was being restricted due to
> known bad firmware encountered in the past. It could be that it would
> be worth adding an option to configure for enabling access to higher
> addresses, alternatively for retaining compatibility with the broken
> systems.
>
> I'm opposed to a config option for this. We don't want to have several
> variants of grub binaries for the same platform. If we want to support
> >4GiB memory we should detect the buggy firmware on runtime. It's pretty
> easy: buggy firmware didn't map memory above 4GiB. We can then either avoid
> memory above 4GiB or map it ourselves.
>
>
> What about a dynamic variable instead or at least accessible from script?
> So a user could redefine a value of any kind.
>
What if advantage compared to automatic detection. And still, I want to
know about usecase

>
>
> > The feature should support UEFI BIOS boot mode.
>
> I do not understand this statement.
>
> /
>     Leif
>
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