qemu memory backend

源码中 ./backends

•  hostmem-epc.c
•  hostmem-file.c
•  hostmem-memfd.c
•  hostmem-ram.c
•  hostmem-shm.c
•  hostmem.c
•  iommufd.c

https://www.qemu.org/docs/master/system/devices/vhost-user.html 中提到，只有这几种 backend 才可以

In order for the daemon to access the VirtIO queues to process the requests it needs access to the guest’s address space. This is achieved via the memory-backend-file, memory-backend-memfd, or memory-backend-shm objects.

基本操作

1. info memdev

(qemu) info memdev
memory backend: mem0
  size:  8589934592
  merge: true
  dump: true
  prealloc: false
  share: true
  reserve: true
  policy: default
  host nodes:

执行流程:

启动的时候:

• main
  • qemu_init
    • qemu_create_late_backends
      • object_option_foreach_add
        • user_creatable_add_qapi
          • user_creatable_add_type
            • user_creatable_complete
              • host_memory_backend_memory_complete
                • memfd_backend_memory_alloc

只会调用一次，而且热插不会调用，看来是通过 maxmem 一次性就分配好了。

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