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| -rw-r--r-- | Kernel/Memory/MemoryManager.cpp | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/Kernel/Memory/MemoryManager.cpp b/Kernel/Memory/MemoryManager.cpp index 015868a391..74bebae9ea 100644 --- a/Kernel/Memory/MemoryManager.cpp +++ b/Kernel/Memory/MemoryManager.cpp @@ -245,6 +245,28 @@ UNMAP_AFTER_INIT void MemoryManager::parse_memory_map() m_global_data.with([&](auto& global_data) { global_data.used_memory_ranges.ensure_capacity(4); #if ARCH(X86_64) + // NOTE: We don't touch the first 1 MiB of RAM on x86-64 even if it's usable as indicated + // by a certain memory map. There are 2 reasons for this: + // + // The first reason is specified for Linux doing the same thing in + // https://cateee.net/lkddb/web-lkddb/X86_RESERVE_LOW.html - + // "By default we reserve the first 64K of physical RAM, as a number of BIOSes are known + // to corrupt that memory range during events such as suspend/resume or monitor cable insertion, + // so it must not be used by the kernel." + // + // Linux also allows configuring this knob in compiletime for this reserved range length, that might + // also include the EBDA and other potential ranges in the first 1 MiB that could be corrupted by the BIOS: + // "You can set this to 4 if you are absolutely sure that you trust the BIOS to get all its memory + // reservations and usages right. If you know your BIOS have problems beyond the default 64K area, + // you can set this to 640 to avoid using the entire low memory range." + // + // The second reason is that the first 1 MiB memory range should also include the actual BIOS blob + // together with possible execution blob code for various option ROMs, which should not be touched + // by our kernel. + // + // **To be completely on the safe side** and never worry about where the EBDA is located, how BIOS might + // corrupt the low memory range during power state changing, other bad behavior of some BIOS might change + // a value in the very first 64k bytes of RAM, etc - we should just ignore this range completely. global_data.used_memory_ranges.append(UsedMemoryRange { UsedMemoryRangeType::LowMemory, PhysicalAddress(0x00000000), PhysicalAddress(1 * MiB) }); #endif global_data.used_memory_ranges.append(UsedMemoryRange { UsedMemoryRangeType::Kernel, PhysicalAddress(virtual_to_low_physical((FlatPtr)start_of_kernel_image)), PhysicalAddress(page_round_up(virtual_to_low_physical((FlatPtr)end_of_kernel_image)).release_value_but_fixme_should_propagate_errors()) }); |