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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Thu, 25 Nov 2021 02:22:45 +1100
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Subject: [PATCH] mm: Document GRUB internal memory management structures
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I spent more than a trivial quantity of time figuring out pre_size and
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whether a memory region's size contains the header cell or not.
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Document the meanings of all the properties. Hopefully now no-one else
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has to figure it out!
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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(cherry picked from commit a6c5c52ccffd2674d43db25fb4baa9c528526aa0)
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---
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include/grub/mm_private.h | 28 ++++++++++++++++++++++++++++
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1 file changed, 28 insertions(+)
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diff --git a/include/grub/mm_private.h b/include/grub/mm_private.h
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index c2c4cb1511..203533cc3d 100644
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--- a/include/grub/mm_private.h
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+++ b/include/grub/mm_private.h
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@@ -21,15 +21,27 @@
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#include <grub/mm.h>
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+/* For context, see kern/mm.c */
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+
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/* Magic words. */
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#define GRUB_MM_FREE_MAGIC 0x2d3c2808
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#define GRUB_MM_ALLOC_MAGIC 0x6db08fa4
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+/* A header describing a block of memory - either allocated or free */
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typedef struct grub_mm_header
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{
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+ /*
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+ * The 'next' free block in this region's circular free list.
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+ * Only meaningful if the block is free.
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+ */
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struct grub_mm_header *next;
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+ /* The block size, not in bytes but the number of cells of
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+ * GRUB_MM_ALIGN bytes. Includes the header cell.
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+ */
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grub_size_t size;
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+ /* either free or alloc magic, depending on the block type. */
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grub_size_t magic;
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+ /* pad to cell size: see the top of kern/mm.c. */
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#if GRUB_CPU_SIZEOF_VOID_P == 4
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char padding[4];
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#elif GRUB_CPU_SIZEOF_VOID_P == 8
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@@ -48,11 +60,27 @@ typedef struct grub_mm_header
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#define GRUB_MM_ALIGN (1 << GRUB_MM_ALIGN_LOG2)
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+/* A region from which we can make allocations. */
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typedef struct grub_mm_region
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{
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+ /* The first free block in this region. */
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struct grub_mm_header *first;
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+
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+ /*
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+ * The next region in the linked list of regions. Regions are initially
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+ * sorted in order of increasing size, but can grow, in which case the
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+ * ordering may not be preserved.
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+ */
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struct grub_mm_region *next;
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+
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+ /*
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+ * A grub_mm_region will always be aligned to cell size. The pre-size is
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+ * the number of bytes we were given but had to skip in order to get that
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+ * alignment.
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+ */
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grub_size_t pre_size;
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+
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+ /* How many bytes are in this region? (free and allocated) */
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grub_size_t size;
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}
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*grub_mm_region_t;
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