In QEMU today, a VM with a vhost-user device can hot add memory a maximum of 8 times. See these threads, among others:
[1] https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg01046.html https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg01236.html [2] https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg04656.html This series introduces a new protocol feature VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS which, when enabled, lifts the restriction on the maximum number RAM slots imposed by vhost-user. The patch consists of 3 changes: 1. Fixed assert in vhost_user_set_mem_table_postcopy: This is a bug fix in the postcopy migration path 2. Refactor vhost_user_set_mem_table functions: This is a non-functional change refractoring the vhost_user_set_mem_table and vhost_user_set_mem_table_postcopy functions such that the feature can be more cleanly added. 3. Lift max memory slots limit imposed by vhost-user: This change introduces the new protocol feature VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS. The implementation details are explained in more detail in the commit messages, but at a high level the new protocol feature works as follows: - If the VHOST_USER_PROTCOL_F_CONFIGURE_SLOTS feature is enabled, QEMU will send multiple VHOST_USER_ADD_MEM_REG and VHOST_USER_REM_MEM_REG messages to map and unmap individual memory regions instead of one large VHOST_USER_SET_MEM_TABLE message containing all memory regions. - The vhost-user struct maintains a ’shadow state’ of memory regions already sent to the guest. Each time vhost_user_set_mem_table is called, the shadow state is compared with the new device state. A VHOST_USER_REM_MEM_REG will be sent for each region in the shadow state not in the device state. Then, a VHOST_USER_ADD_MEM_REG will be sent for each region in the device state but not the shadow state. After these messages have been sent, the shadow state will be updated to reflect the new device state. The VHOST_USER_SET_MEM_TABLE message was not reused because as the number of regions grows, the message becomes very large. In practice, such large messages caused problems (truncated messages) and in the past it seems the community has opted for smaller fixed size messages where possible. VRINGs, for example, are sent to the backend individually instead of in one massive message. Current Limitations: - postcopy migration is not supported when the VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS has been negotiated. - VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS cannot be negotiated when VHOST_USER_PROTOCOL_F_REPLY_ACK has also been negotiated. Both of these limitations are due to resource contraints. They are not imposed for technical reasons. Changes since V1: * Kept the assert in vhost_user_set_mem_table_postcopy, but moved it to prevent corruption * Made QEMU send a single VHOST_USER_GET_MAX_MEMSLOTS message at startup and cache the returned value so that QEMU does not need to query the backend every time vhost_backend_memslots_limit is called. Best, Raphael Raphael Norwitz (3): Fixed assert in vhost_user_set_mem_table_postcopy Refactor vhost_user_set_mem_table functions Lift max memory slots limit imposed by vhost-user docs/interop/vhost-user.rst | 43 +++++ hw/virtio/vhost-user.c | 385 +++++++++++++++++++++++++++++++++----------- 2 files changed, 336 insertions(+), 92 deletions(-) -- 1.8.3.1
