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

From: Michel Hermier
Subject: Re: Grub2: add UEFI support for accessing memory address above 4GB.
Date: Tue, 7 Mar 2017 18:08:48 +0100

Le 7 mars 2017 17:24, "Vladimir 'phcoder' Serbinenko" <address@hidden> a écrit :

On Tue, Mar 7, 2017, 08:15 Leif Lindholm <address@hidden> 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
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.

> The feature should support UEFI BIOS boot mode.

I do not understand this statement.


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