Re: [PATCH v2] x86/boot/e820: Re-enable fallback if e820 table is empty

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Thu, Apr 16, 2026 at 02:57:43PM +0800, David Gow wrote:
> In commit 157266edcc56 ("x86/boot/e820: Simplify append_e820_table() and
> remove restriction on single-entry tables"), 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().
> 
> In so doing, we also update the return values to be proper error codes,
> with -ENOENT for this case (there are no entries), and -EINVAL for the case
> where an entry appears invalid. Given none of the callers check the actual
> value -- just whether it's nonzero -- this is largely aesthetic in
> practice.
> 
> Tested against linld, and the kernel boots again fine.

...

> Changes since v2:
> https://lore.kernel.org/lkml/20260415003021.1543723-1-david@davidgow.net/
> - Return -ENOENT instead of -1
> - Return -EINVAL instead of -1 for the case where an entry looks invalid due to
>   an overflow (Thanks, Andy)

Oh, I didn't noticed it's overflow condition, so -ERANGE may suit better.
Whatever, the change looks good enough and it's up to you to follow the above.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>

-- 
With Best Regards,
Andy Shevchenko