On Wed, Jan 15, 2020 at 09:57:03PM -0500, Raphael Norwitz wrote: > 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.
I'm a bit confused about what happens on reconnect. Can you clarify pls? > 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
