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Kernel/Memory: Explain better why we don't use the first 1 MiB on x86_64

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Liav A 2023-04-05 05:25:06 +03:00 • committed by Andrew Kaster
parent ad5abc9a8f
commit fb8d4b7032
1 changed files with 22 additions and 0 deletions
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22
Kernel/Memory/MemoryManager.cpp
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@ -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()) });