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

[David Gow]

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().

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")
Signed-off-by: David Gow <david@xxxxxxxxxxxx>
---
 arch/x86/kernel/e820.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 2a9992758933..7dfd4bdc95fb 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -450,6 +450,10 @@ __init static int append_e820_table(struct boot_e820_entry *entries, u32 nr_entr
 {
 	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;
+
 	while (nr_entries) {
 		u64 start = entry->addr;
 		u64 size  = entry->size;
-- 
2.53.0