[RFC PATCH 3/9] x86/efi: Omit RSRV_KERN memblock reservations when freeing boot regions

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Instead of freeing all EFI boot services code and data regions that were
preliminarily reserved during early boot to work around buggy firmware,
take care to only free those parts that are not marked as
MEMBLOCK_RSRV_KERN. This marking is used by the generic implementation
of efi_mem_reserve() to mark things like informational tables that are
provided to the OS by the firmware, but where the contents of memory
have no significance to the firmware itself. Such assets are often
passed in a EFI boot service data region, leaving it to the OS to decide
whether it needs to be reserved or not.

This removes the need to mark such regions as EFI_MEMORY_RUNTIME, which
is a hack that results in a lot of complexity in updating and
re-allocating the EFI memory map, which would otherwise not need to be
modified at all. Note that x86 is the only EFI arch that does any of
this, others just treat the EFI memory map as immutable.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/platform/efi/quirks.c | 40 +++++++++++++++++---
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
index 35caa5746115..f896930cecda 100644
--- a/arch/x86/platform/efi/quirks.c
+++ b/arch/x86/platform/efi/quirks.c
@@ -536,6 +536,40 @@ void __init efi_unmap_boot_services(void)
 	}
 }
 
+static unsigned long __init
+efi_free_unreserved_subregions(u64 range_start, u64 range_end)
+{
+	struct memblock_region *region;
+	unsigned long freed = 0;
+
+	for_each_reserved_mem_region(region) {
+		u64 region_end = region->base + region->size;
+		u64 start, end;
+
+		/* memblock tables are sorted so no need to carry on */
+		if (region->base >= range_end)
+			break;
+
+		if (region_end < range_start)
+			continue;
+
+		if (region->flags & MEMBLOCK_RSRV_KERN)
+			continue;
+
+		start = PAGE_ALIGN(max(range_start, region->base));
+		end = PAGE_ALIGN_DOWN(min(range_end, region_end));
+
+		if (start >= end)
+			continue;
+
+		free_reserved_area(phys_to_virt(start),
+				   phys_to_virt(end), -1, NULL);
+		freed += (end - start);
+	}
+
+	return freed;
+}
+
 static int __init efi_free_boot_services(void)
 {
 	struct efi_freeable_range *range = ranges_to_free;
@@ -545,11 +579,7 @@ static int __init efi_free_boot_services(void)
 		return 0;
 
 	while (range->start) {
-		void *start = phys_to_virt(range->start);
-		void *end = phys_to_virt(range->end);
-
-		free_reserved_area(start, end, -1, NULL);
-		freed += (end - start);
+		freed += efi_free_unreserved_subregions(range->start, range->end);
 		range++;
 	}
 	kfree(ranges_to_free);
-- 
2.53.0.473.g4a7958ca14-goog