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Although accesses to ram_list.dirty_memory[] use atomics so multiple threads can safely dirty the bitmap, the data structure is not fully thread-safe yet. This patch handles the RAM hotplug case where ram_list.dirty_memory[] is grown. ram_list.dirty_memory[] is change from a regular bitmap to an RCU array of pointers to fixed-size bitmap blocks. Threads can continue accessing bitmap blocks while the array is being extended. See the comments in the code for an in-depth explanation of struct DirtyMemoryBlocks. I have tested that live migration with virtio-blk dataplane works. Backports commit 5b82b703b69acc67b78b98a5efc897a3912719eb from qemu
51 lines
1.7 KiB
C
51 lines
1.7 KiB
C
#ifndef RAMLIST_H
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#define RAMLIST_H
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#include "qemu/queue.h"
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#include "qemu/thread.h"
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#define DIRTY_MEMORY_CODE 0
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#define DIRTY_MEMORY_NUM 1 /* num of dirty bits */
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/* The dirty memory bitmap is split into fixed-size blocks to allow growth
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* under RCU. The bitmap for a block can be accessed as follows:
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*
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* rcu_read_lock();
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*
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* DirtyMemoryBlocks *blocks =
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* atomic_rcu_read(&ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]);
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*
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* ram_addr_t idx = (addr >> TARGET_PAGE_BITS) / DIRTY_MEMORY_BLOCK_SIZE;
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* unsigned long *block = blocks.blocks[idx];
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* ...access block bitmap...
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*
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* rcu_read_unlock();
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*
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* Remember to check for the end of the block when accessing a range of
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* addresses. Move on to the next block if you reach the end.
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*
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* Organization into blocks allows dirty memory to grow (but not shrink) under
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* RCU. When adding new RAMBlocks requires the dirty memory to grow, a new
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* DirtyMemoryBlocks array is allocated with pointers to existing blocks kept
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* the same. Other threads can safely access existing blocks while dirty
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* memory is being grown. When no threads are using the old DirtyMemoryBlocks
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* anymore it is freed by RCU (but the underlying blocks stay because they are
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* pointed to from the new DirtyMemoryBlocks).
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*/
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#define DIRTY_MEMORY_BLOCK_SIZE ((ram_addr_t)256 * 1024 * 8)
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typedef struct {
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//struct rcu_head rcu;
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// Unicorn: unicorn-specific variable to make memory handling less painful
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size_t num_blocks;
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unsigned long *blocks[];
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} DirtyMemoryBlocks;
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typedef struct RAMList {
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RAMBlock *mru_block;
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QLIST_HEAD(, RAMBlock) blocks;
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DirtyMemoryBlocks *dirty_memory[DIRTY_MEMORY_NUM];
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uint32_t version;
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} RAMList;
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#endif
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