[PATCH v8 12/17] x86/e820: temporarily enable KHO scratch for memory below 1M

Changyuan Lyu
2025-05-09T07:47:17Z

From: Alexander Graf <graf@amazon.com>

KHO kernels are special and use only scratch memory for memblock
allocations, but memory below 1M is ignored by kernel after early boot
and cannot be naturally marked as scratch.

To allow allocation of the real-mode trampoline and a few (if any) other
very early allocations from below 1M forcibly mark the memory below 1M
as scratch.

After real mode trampoline is allocated, clear that scratch marking.

Signed-off-by: Alexander Graf <graf@amazon.com>
Co-developed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Co-developed-by: Changyuan Lyu <changyuanl@google.com>
Signed-off-by: Changyuan Lyu <changyuanl@google.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
---
arch/x86/kernel/e820.c | 18 ++++++++++++++++++
arch/x86/realmode/init.c | 2 ++
2 files changed, 20 insertions(+)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 9920122018a0b..c3acbd26408ba 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1299,6 +1299,24 @@ void __init e820__memblock_setup(void)
memblock_add(entry->addr, entry->size);
}

+ /*
+ * At this point memblock is only allowed to allocate from memory
+ * below 1M (aka ISA_END_ADDRESS) up until direct map is completely set
+ * up in init_mem_mapping().
+ *
+ * KHO kernels are special and use only scratch memory for memblock
+ * allocations, but memory below 1M is ignored by kernel after early
+ * boot and cannot be naturally marked as scratch.
+ *
+ * To allow allocation of the real-mode trampoline and a few (if any)
+ * other very early allocations from below 1M forcibly mark the memory
+ * below 1M as scratch.
+ *
+ * After real mode trampoline is allocated, we clear that scratch
+ * marking.
+ */
+ memblock_mark_kho_scratch(0, SZ_1M);
+
/*
* 32-bit systems are limited to 4BG of memory even with HIGHMEM and
* to even less without it.
diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index f9bc444a3064d..9b9f4534086d2 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -65,6 +65,8 @@ void __init reserve_real_mode(void)
* setup_arch().
*/
memblock_reserve(0, SZ_1M);
+
+ memblock_clear_kho_scratch(0, SZ_1M);
}

static void __init sme_sev_setup_real_mode(struct trampoline_header *th)
--
2.49.0.1015.ga840276032-goog