Re: [PATCH 059/112] efi: payload: init: restrict barebox mem to first 1G only on x86

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Michael,

On 04.01.24 19:10, Michael Olbrich wrote:
> On Thu, Jan 04, 2024 at 12:17:12PM +0100, Ahmad Fatoum wrote:
>> On 03.01.24 19:58, Michael Olbrich wrote:
>>> On Wed, Jan 03, 2024 at 07:12:19PM +0100, Ahmad Fatoum wrote:
>>>> When run as EFI payload, barebox will not have full access to DRAM and will
>>>> have to ask the EFI firmware for memory. It does so once at the beginning
>>>> and since commit 356aaef5a37d ("efi: improve malloc pool allocation"),
>>>> the buffer was always placed beneath the 1G boundary.
>>>>
>>>> Aforementioned commit didn't elaborate why a maximum of 1G in particular
>>>> was chosen. Anyways, non-PC architectures have different memory maps, so
>>>> a 1G limit doesn't make sense there. Therefore restrict the limit to
>>>> ARCH_X86. On non-x86, we allow the memory pages to be located anywhere.
>>>
>>> The 1G is somewhat arbitrary but if you don't restrict the memory to 32bit,
>>> then loading the kernel may fail: The addresses in "struct
>>> linux_kernel_header" are only 32bit. So if you impose no restrictions at
>>> all, then this memory cannot be used for the kernel and initrd.
>>
>> The limit for x86 remains at 1G. It's only for other architectures, that we
>> remove the limit. I suspect that other architectures won't have this kind
>> of problems though , because they would boot the kernel image via EFI load
>> and start image boot services, where the EFI firmware takes care to layout
>> the boot artifacts.
>>
>> Once we have that in place, we could even use it on x86 instead of
>> using struct linux_kernel_header, but I haven't investigated this yet.
> 
> How do you load the initrd via EFI? As far as I know, that only works as a
> filename in the EFI boot partition.

This seems to be relaxed in newer kernels, see
https://lore.kernel.org/linux-efi/20200207202637.GA3464906@rani.riverdale.lan/T/

> That was the reason for implementing
> the "legacy" boot mode. And what about the device-tree?

There is an EFI config table for FDT (DEVICE_TREE_GUID in Linux code). barebox
as EFI loader can use it to provide its own DT. I have to check what's the cleanest
way for barebox as EFI payload to provide a different DT, but I hope there is a way
without barebox as EFI payload having to manipulate the device tree itself.

Cheers,
Ahmad

> 
> Michael
> 
>>>> This can break PCI drivers on systems that so far assumed barebox memory
>>>> is located in the first 4G, but that's ok. That code will need to be
>>>> fixed eventually anyway.
>>>>
>>>> Cc: Michael Olbrich <m.olbrich@pengutronix.de>
>>>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>>>> ---
>>>>  efi/payload/init.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/efi/payload/init.c b/efi/payload/init.c
>>>> index 4b612e61afb3..0c2f38e9c86a 100644
>>>> --- a/efi/payload/init.c
>>>> +++ b/efi/payload/init.c
>>>> @@ -292,7 +292,7 @@ void efi_main(efi_handle_t image, struct efi_system_table *sys_table)
>>>>  		BS->handle_protocol(efi_loaded_image->device_handle,
>>>>  				&efi_device_path_protocol_guid, (void **)&efi_device_path);
>>>>  
>>>> -	mem = 0x3fffffff;
>>>> +	mem = IS_ENABLED(CONFIG_X86) ? 0x3fffffff : ~0ULL;
>>>>  	for (memsize = SZ_256M; memsize >= SZ_8M; memsize /= 2) {
>>>>  		efiret  = BS->allocate_pages(EFI_ALLOCATE_MAX_ADDRESS,
>>>>  					     EFI_LOADER_DATA,
>>>> -- 
>>>> 2.39.2
>>>>
>>>>
>>>
>>
>> -- 
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>>
>>
> 

-- 
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