UEFI 2.8 defines an EFI_MEMORY_SP attribute bit to augment the interpretation of the EFI Memory Types as "reserved for a specific purpose". The intent of this bit is to allow the OS to identify precious or scarce memory resources and optionally manage it separately from EfiConventionalMemory. As defined older OSes that do not know about this attribute are permitted to ignore it and the memory will be handled according to the OS default policy for the given memory type. In other words, this "specific purpose" hint is deliberately weaker than EfiReservedMemoryType in that the system continues to operate if the OS takes no action on the attribute. The risk of taking no action is potentially unwanted / unmovable kernel allocations from the designated resource that prevent the full realization of the "specific purpose". For example, consider a system with a high-bandwidth memory pool. Older kernels are permitted to boot and consume that memory as conventional "System-RAM" newer kernels may arrange for that memory to be set aside (soft reserved) by the system administrator for a dedicated high-bandwidth memory aware application to consume. Specifically, this mechanism allows for the elimination of scenarios where platform firmware tries to game OS policy by lying about ACPI SLIT values, i.e. claiming that a precious memory resource has a high distance to trigger the OS to avoid it by default. This reservation hint allows platform-firmware to instead tell the truth about performance characteristics by indicate to OS memory management to put immovable allocations elsewhere. Implement simple detection of the bit for EFI memory table dumps and save the kernel policy for a follow-on change.
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| @@ -112,6 +112,7 @@ typedef struct { · 111 unmodified lines | ||
| 112 | 112 | #define EFI_MEMORY_MORE_RELIABLE \ |
| 113 | 113 | ((u64)0x0000000000010000ULL) /* higher reliability */ |
| 114 | 114 | #define EFI_MEMORY_RO ((u64)0x0000000000020000ULL) /* read-only */ |
| 115 | +#define EFI_MEMORY_SP ((u64)0x0000000000040000ULL) /* soft reserved */ | |
| 115 | 116 | #define EFI_MEMORY_RUNTIME ((u64)0x8000000000000000ULL) /* range requires runtime mapping */ |
| 116 | 117 | #define EFI_MEMORY_DESCRIPTOR_VERSION 1 |
| 117 | 118 | |