[kvm-unit-tests PATCH 2/8] x86: fix EFI memory allocator to clamp regions to 4 GiB

[Giacomo Mazzola <gmazz@amazon.de>]

setup_page_table() creates an identity map covering only the first
4 GiB (4 PDPT entries × 512 PDE entries × 2 MiB pages).  However,
setup_memory_allocator() picks the largest EFI_CONVENTIONAL_MEMORY
region without regard to this limit.

On machines with more than 4 GiB of RAM, the largest conventional
memory region often starts below 4 GiB but extends well above it,
or resides entirely above 4 GiB.  When the allocator selects such a
region, any access to memory beyond the mapped range triggers a page
fault, crashing the test before it can run.

Fix this by clamping every candidate region to the 4 GiB boundary:
skip regions that start at or above 4 GiB entirely, and truncate
regions that straddle the boundary so only the mapped portion is
considered.  The allocator then picks the largest usable region that
is fully covered by the identity map.

Signed-off-by: Giacomo Mazzola <gmazz@amazon.de>
---
 lib/x86/setup.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/lib/x86/setup.c b/lib/x86/setup.c
index c2f1c6d0..09241b33 100644
--- a/lib/x86/setup.c
+++ b/lib/x86/setup.c
@@ -215,6 +215,7 @@ static efi_status_t setup_memory_allocator(efi_bootinfo_t *efi_bootinfo)
 	struct efi_boot_memmap *map = &(efi_bootinfo->mem_map);
 	efi_memory_desc_t *buffer = *map->map;
 	efi_memory_desc_t *d = NULL;
+	unsigned long max_addr = 1UL << 32;
 
 	/*
 	 * The 'buffer' contains multiple descriptors that describe memory
@@ -222,13 +223,26 @@ static efi_status_t setup_memory_allocator(efi_bootinfo_t *efi_bootinfo)
 	 * EFI_CONVENTIONAL_MEMORY region which will be used to set up the
 	 * memory allocator, so that the memory allocator can work in the
 	 * largest free continuous memory region.
+	 *
+	 * Regions are clamped to the first 4 GiB because the EFI page
+	 * tables set up by setup_page_table() only map that range.
 	 */
 	for (i = 0; i < *(map->map_size); i += *(map->desc_size)) {
 		d = (efi_memory_desc_t *)(&((u8 *)buffer)[i]);
 		if (d->type == EFI_CONVENTIONAL_MEMORY) {
-			if (free_mem_pages < d->num_pages) {
-				free_mem_pages = d->num_pages;
-				free_mem_start = d->phys_addr;
+			unsigned long start = d->phys_addr;
+			unsigned long end = start + (d->num_pages << EFI_PAGE_SHIFT);
+			unsigned long pages;
+
+			if (start >= max_addr)
+				continue;
+			if (end > max_addr)
+				end = max_addr;
+			pages = (end - start) >> EFI_PAGE_SHIFT;
+
+			if (free_mem_pages < pages) {
+				free_mem_pages = pages;
+				free_mem_start = start;
 			}
 		}
 	}
-- 
2.47.3




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