[PATCH] efi: Support booting with kexec handover (KHO)

[Evangelos Petrongonas <epetron@amazon.de>]

When KHO (Kexec HandOver) is enabled, it sets up scratch memory regions
early during device tree scanning. After kexec, the new kernel
exclusively uses this region for memory allocations during boot up to
the initialization of the page allocator

However, when booting with EFI, EFI's reserve_regions() uses
memblock_remove(0, PHYS_ADDR_MAX) to clear all memory regions before
rebuilding them from EFI data. This destroys KHO scratch regions and
their flags, thus causing a kernel panic, as there are no scratch
memory regions.

Instead of wholesale removal, iterate through memory regions and only
remove non-KHO ones. This preserves KHO scratch regions while still
allowing EFI to rebuild its memory map.

Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
---

Reproduction/Verification Steps
The issue and the fix can be reproduced/verified by booting a VM with
EFI and attempting to perform a KHO enabled kexec. The fix
was developed/tested on arm64.

 drivers/firmware/efi/efi-init.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/efi/efi-init.c b/drivers/firmware/efi/efi-init.c
index a00e07b853f22..2f08b1ab764f6 100644
--- a/drivers/firmware/efi/efi-init.c
+++ b/drivers/firmware/efi/efi-init.c
@@ -164,12 +164,35 @@ static __init void reserve_regions(void)
 		pr_info("Processing EFI memory map:\n");
 
 	/*
-	 * Discard memblocks discovered so far: if there are any at this
-	 * point, they originate from memory nodes in the DT, and UEFI
-	 * uses its own memory map instead.
+	 * Discard memblocks discovered so far except for KHO scratch regions.
+	 * Most memblocks at this point originate from memory nodes in the DT,
+	 * and UEFI uses its own memory map instead. However, if KHO is enabled,
+	 * scratch regions must be preserved.
 	 */
 	memblock_dump_all();
-	memblock_remove(0, PHYS_ADDR_MAX);
+
+	if (IS_ENABLED(CONFIG_MEMBLOCK_KHO_SCRATCH)) {
+		struct memblock_region *reg;
+		phys_addr_t start, size;
+		int i;
+
+		/* Remove all non-KHO regions */
+		for (i = memblock.memory.cnt - 1; i >= 0; i--) {
+			reg = &memblock.memory.regions[i];
+			if (!memblock_is_kho_scratch(reg)) {
+				start = reg->base;
+				size = reg->size;
+				memblock_remove(start, size);
+			}
+		}
+	} else {
+	/*
+	 * KHO is disabled. Discard memblocks discovered so far: if there
+	 * are any at this point, they originate from memory nodes in the
+	 * DT, and UEFI uses its own memory map instead.
+	 */
+		memblock_remove(0, PHYS_ADDR_MAX);
+	}
 
 	for_each_efi_memory_desc(md) {
 		paddr = md->phys_addr;
-- 
2.43.0




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