# Copyright (c) 2021 Intel Corporation # # SPDX-License-Identifier: Apache-2.0 menu "Virtual Memory Support" config KERNEL_VM_SUPPORT bool help Hidden option to enable virtual memory Kconfigs. if KERNEL_VM_SUPPORT DT_CHOSEN_Z_SRAM := zephyr,sram config KERNEL_VM_BASE hex "Virtual address space base address" default $(dt_chosen_reg_addr_hex,$(DT_CHOSEN_Z_SRAM)) help Define the base of the kernel's address space. By default, this is the same as the DT_CHOSEN_Z_SRAM physical base SRAM address from DTS, in which case RAM will be identity-mapped. Some architectures may require RAM to be mapped in this way; they may have just one RAM region and doing this makes linking much simpler, as at least when the kernel boots all virtual RAM addresses are the same as their physical address (demand paging at runtime may later modify this for non-pinned page frames). Otherwise, if RAM isn't identity-mapped: 1. It is the architecture's responsibility to transition the instruction pointer to virtual addresses at early boot before entering the kernel at z_cstart(). 2. The underlying architecture may impose constraints on the bounds of the kernel's address space, such as not overlapping physical RAM regions if RAM is not identity-mapped, or the virtual and physical base addresses being aligned to some common value (which allows double-linking of paging structures to make the instruction pointer transition simpler). Zephyr does not implement a split address space and if multiple page tables are in use, they all have the same virtual-to-physical mappings (with potentially different permissions). config KERNEL_VM_OFFSET hex "Kernel offset within address space" default 0 help Offset that the kernel image begins within its address space, if this is not the same offset from the beginning of RAM. Some care may need to be taken in selecting this value. In certain build-time cases, or when a physical address cannot be looked up in page tables, the equation: virt = phys + ((KERNEL_VM_BASE + KERNEL_VM_OFFSET) - (SRAM_BASE_ADDRESS + SRAM_OFFSET)) Will be used to convert between physical and virtual addresses for memory that is mapped at boot. This uncommon and is only necessary if the beginning of VM and physical memory have dissimilar alignment. config KERNEL_VM_SIZE hex "Size of kernel address space in bytes" default 0x800000 help Size of the kernel's address space. Constraining this helps control how much total memory can be used for page tables. The difference between KERNEL_VM_BASE and KERNEL_VM_SIZE indicates the size of the virtual region for runtime memory mappings. This is needed for mapping driver MMIO regions, as well as special RAM mapping use-cases such as VSDO pages, memory mapped thread stacks, and anonymous memory mappings. The kernel itself will be mapped in here as well at boot. Systems with very large amounts of memory (such as 512M or more) will want to use a 64-bit build of Zephyr, there are no plans to implement a notion of "high" memory in Zephyr to work around physical RAM size larger than the defined bounds of the virtual address space. config KERNEL_DIRECT_MAP bool "Memory region direct-map support" depends on MMU help This enables the direct-map support, namely the region can be 1:1 mapping between virtual address and physical address. If the specific memory region is in the virtual memory space and there isn't overlap with the existed mappings, it will reserve the region from the virtual memory space and do the mapping, otherwise it will fail. And any attempt across the boundary of the virtual memory space will fail. Note that this is for compatibility and portable apps shouldn't be using it. endif # KERNEL_VM_SUPPORT menuconfig MMU bool "MMU features" depends on CPU_HAS_MMU select KERNEL_VM_SUPPORT help This option is enabled when the CPU's memory management unit is active and the arch_mem_map() API is available. if MMU config MMU_PAGE_SIZE hex "Size of smallest granularity MMU page" default 0x1000 help Size of memory pages. Varies per MMU but 4K is common. For MMUs that support multiple page sizes, put the smallest one here. menuconfig DEMAND_PAGING bool "Demand paging [EXPERIMENTAL]" depends on ARCH_HAS_DEMAND_PAGING help Enable demand paging. Requires architecture support in how the kernel is linked and the implementation of an eviction algorithm and a backing store for evicted pages. if DEMAND_PAGING config DEMAND_PAGING_ALLOW_IRQ bool "Allow interrupts during page-ins/outs" help Allow interrupts to be serviced while pages are being evicted or retrieved from the backing store. This is much better for system latency, but any code running in interrupt context that page faults will cause a kernel panic. Such code must work with exclusively pinned code and data pages. The scheduler is still disabled during this operation. If this option is disabled, the page fault servicing logic runs with interrupts disabled for the entire operation. However, ISRs may also page fault. config DEMAND_PAGING_PAGE_FRAMES_RESERVE int "Number of page frames reserved for paging" default 32 if !LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT default 0 help This sets the number of page frames that will be reserved for paging that do not count towards free memory. This is to ensure that there are some page frames available for paging code and data. Otherwise, it would be possible to exhaust all page frames via anonymous memory mappings. config DEMAND_PAGING_STATS bool "Gather Demand Paging Statistics" help This enables gathering various statistics related to demand paging, e.g. number of pagefaults. This is useful for tuning eviction algorithms and optimizing backing store. Should say N in production system as this is not without cost. config DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS bool "Use Timing Functions to Gather Demand Paging Statistics" select TIMING_FUNCTIONS_NEED_AT_BOOT help Use timing functions to gather various demand paging statistics. config DEMAND_PAGING_THREAD_STATS bool "Gather per Thread Demand Paging Statistics" depends on DEMAND_PAGING_STATS help This enables gathering per thread statistics related to demand paging. Should say N in production system as this is not without cost. config DEMAND_PAGING_TIMING_HISTOGRAM bool "Gather Demand Paging Execution Timing Histogram" depends on DEMAND_PAGING_STATS help This gathers the histogram of execution time on page eviction selection, and backing store page in and page out. Should say N in production system as this is not without cost. config DEMAND_PAGING_TIMING_HISTOGRAM_NUM_BINS int "Number of bins (buckets) in Demand Paging Timing Histogram" depends on DEMAND_PAGING_TIMING_HISTOGRAM default 10 help Defines the number of bins (buckets) in the histogram used for gathering execution timing information for demand paging. This requires k_mem_paging_eviction_histogram_bounds[] and k_mem_paging_backing_store_histogram_bounds[] to define the upper bounds for each bin. See kernel/statistics.c for information. endif # DEMAND_PAGING endif # MMU endmenu # Virtual Memory Support