Re: [RFC PATCH 0/3] vhost-user: Lift Max Ram Slots Limitation

[Raphael Norwitz <raphael.norwitz@nutanix.com>]

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On Mon, Dec 09, 2019 at 02:00:44AM -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 RFC/patch set 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 Error Handling in vhost_user_set_mem_table_postcopy:
>    This is a bug fix in the postcopy migration path
> 2. vhost-user: 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. Introduce Configurable Number of Memory Slots Exposed 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.
> 
> Questions:
> - In the event transmitting a VHOST_USER_ADD_MEM_REG or
>   VHOST_USER_REM_REG message fails, is there any reason the error handling
>   should differ from when transmitting VHOST_USER_SET_MEM_TABLE message fails?
> - Is there a cleaner way to ensure to ensure a postcopy migration cannot be
>   started with this protocol feature enabled?
> 
> Best,
> Raphael
> 
> Raphael Norwitz (3):
>   Fixed Error Handling in vhost_user_set_mem_table_postcopy
>   vhost-user: Refactor vhost_user_set_mem_table Functions
>   Introduce Configurable Number of Memory Slots Exposed by vhost-user:
> 
>  docs/interop/vhost-user.rst |  43 +++++
>  hw/virtio/vhost-user.c      | 384 +++++++++++++++++++++++++++++++++-----------
>  2 files changed, 335 insertions(+), 92 deletions(-)
> 
> -- 
> 1.8.3.1
> 
>