132 lines
4.0 KiB
C
132 lines
4.0 KiB
C
/*
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* Copyright (c) 2020 Intel Corporation
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*
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* SPDX-License-Identifier: Apache-2.0
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*
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* RAM-based memory buffer backing store implementation for demo purposes
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*/
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#include <mmu.h>
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#include <string.h>
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#include <kernel_arch_interface.h>
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/*
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* TODO:
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*
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* This is a demonstration backing store for testing the kernel side of the
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* demand paging feature. In production there are basically two types of
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* backing stores:
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*
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* 1) A large, sparse backing store that is big enough to capture the entire
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* address space. Implementation of these is very simple; the location
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* token is just a function of the evicted virtual address and no space
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* management is necessary. Clean copies of paged-in data pages may be kept
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* indefinitely.
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*
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* 2) A backing store that has limited storage space, and is not sufficiently
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* large to hold clean copies of all mapped memory.
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*
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* This backing store is an example of the latter case. However, locations
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* are freed as soon as pages are paged in, in z_backing_store_page_finalize().
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* This implies that all data pages are treated as dirty as
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* Z_PAGE_FRAME_BACKED is never set, even if the data page was paged out before
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* and not modified since then.
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*
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* An optimization a real backing store will want is have
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* z_backing_store_page_finalize() note the storage location of a paged-in
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* data page in a custom field of its associated z_page_frame, and set the
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* Z_PAGE_FRAME_BACKED bit. Invocations of z_backing_store_location_get() will
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* have logic to return the previous clean page location instead of allocating
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* a new one if Z_PAGE_FRAME_BACKED is set.
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*
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* This will, however, require the implementation of a clean page
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* eviction algorithm, to free backing store locations for loaded data pages
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* as the backing store fills up, and clear the Z_PAGE_FRAME_BACKED bit
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* appropriately.
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*
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* All of this logic is local to the backing store implementation; from the
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* core kernel's perspective the only change is that Z_PAGE_FRAME_BACKED
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* starts getting set for certain page frames after a page-in (and possibly
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* cleared at a later time).
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*/
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static char backing_store[CONFIG_MMU_PAGE_SIZE *
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CONFIG_BACKING_STORE_RAM_PAGES];
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static struct k_mem_slab backing_slabs;
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static unsigned int free_slabs;
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static void *location_to_slab(uintptr_t location)
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{
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__ASSERT(location % CONFIG_MMU_PAGE_SIZE == 0,
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"unaligned location 0x%lx", location);
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__ASSERT(location <
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(CONFIG_BACKING_STORE_RAM_PAGES * CONFIG_MMU_PAGE_SIZE),
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"bad location 0x%lx, past bounds of backing store", location);
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return backing_store + location;
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}
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static uintptr_t slab_to_location(void *slab)
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{
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char *pos = slab;
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uintptr_t offset;
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__ASSERT(pos >= backing_store &&
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pos < backing_store + ARRAY_SIZE(backing_store),
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"bad slab pointer %p", slab);
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offset = pos - backing_store;
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__ASSERT(offset % CONFIG_MMU_PAGE_SIZE == 0,
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"unaligned slab pointer %p", slab);
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return offset;
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}
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int z_backing_store_location_get(struct z_page_frame *pf, uintptr_t *location,
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bool page_fault)
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{
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int ret;
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void *slab;
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if ((!page_fault && free_slabs == 1) || free_slabs == 0) {
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return -ENOMEM;
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}
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ret = k_mem_slab_alloc(&backing_slabs, &slab, K_NO_WAIT);
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__ASSERT(ret == 0, "slab count mismatch");
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(void)ret;
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*location = slab_to_location(slab);
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free_slabs--;
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return 0;
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}
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void z_backing_store_location_free(uintptr_t location)
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{
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void *slab = location_to_slab(location);
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k_mem_slab_free(&backing_slabs, &slab);
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free_slabs++;
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}
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void z_backing_store_page_out(uintptr_t location)
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{
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(void)memcpy(location_to_slab(location), Z_SCRATCH_PAGE,
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CONFIG_MMU_PAGE_SIZE);
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}
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void z_backing_store_page_in(uintptr_t location)
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{
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(void)memcpy(Z_SCRATCH_PAGE, location_to_slab(location),
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CONFIG_MMU_PAGE_SIZE);
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}
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void z_backing_store_page_finalize(struct z_page_frame *pf, uintptr_t location)
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{
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z_backing_store_location_free(location);
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}
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void z_backing_store_init(void)
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{
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k_mem_slab_init(&backing_slabs, backing_store, CONFIG_MMU_PAGE_SIZE,
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CONFIG_BACKING_STORE_RAM_PAGES);
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free_slabs = CONFIG_BACKING_STORE_RAM_PAGES;
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}
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