Re: [PATCH] x86/boot/e820: Re-enable fallback if e820 table is empty

[Andy Shevchenko]

On Wed, Apr 15, 2026 at 08:30:18AM +0800, David Gow wrote:
> In commit 157266edcc56 ("x86/boot/e820: Simplify append_e820_table() and remove restriction on single-entry tables"),

Note, it's fine to wrap this line in the commit message, it's only requirement
to tags to be "1 tag — 1 line".

> the check that the number of entries in the e820 table was removed. The
> intention was to support single-entry maps, but by removing the check
> entirely, we also skip the fallback (to, e.g., the BIOS 88h function).
> 
> This means that if no E820 map is passed in from the bootloader (which is
> the case on some bootloaders, like linld), we end up with an empty memory
> map, and the kernel fails to boot (either by deadlocking on OOM, or by
> failing to allocate the real mode trampoline, or similar).
> 
> Re-instate the check in append_e820_table(), but only check nr_entries is
> non-zero. This allows e820__memory_setup_default() to fall back to other
> memory size sources, and doesn't affect e820__memory_setup_extended(), as
> the latter ignores the return value from append_e820_table().
> 
> Tested against linld, and the kernel boots again fine.
> 
> Fixes: 157266edcc56 ("x86/boot/e820: Simplify append_e820_table() and remove restriction on single-entry tables")

...

>  	struct boot_e820_entry *entry = entries;
>  
> +	/* If there aren't any entries, we'll want to fall-back to another source. */
> +	if (!nr_entries)
> +		return -1;

Can we return -ENODATA or -ENOENT here?

-- 
With Best Regards,
Andy Shevchenko