Re: [PATCH v4 00/10] vhost-user: Lift Max Ram Slots Limitation

[Raphael Norwitz]

ping

On Thu, May 21, 2020 at 1:00 AM Raphael Norwitz
<raphael.norwitz@nutanix.com> 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.
>
> Without vhost-user, a Qemu VM can support 256 ram slots (for ACPI targets),
> or potentially more (the KVM max is 512). With each region, a file descriptor
> must be sent over the socket. If that many regions are sent in a single 
> message
> there could be upwards of 256 file descriptors being opened in the backend 
> process
> at once. Opening that many fds could easily push the process past the open fd 
> limit,
> especially considering one backend process could have multiple vhost threads,
> exposing different devices to different Qemu instances. Therefore to safely 
> lift the
> limit, transmitting regions should be split up over multiple messages.
>
> In addition, 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.
>
> 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_MEM_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 series consists of 10 changes:
> 1. Add helper to populate vhost-user message regions:
>     This change adds a helper to populate a VhostUserMemoryRegion from a
>     struct vhost_memory_region, which needs to be done in multiple places in
>     in this series.
>
> 2. Add vhost-user helper to get MemoryRegion data
>     This changes adds a helper to get a pointer to a MemoryRegion struct, 
> along
>     with it's offset address and associated file descriptor. This helper is 
> used to
>     simplify other vhost-user code paths and will be needed elsewhere in this
>     series.
>
> 3. Add VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>     This change adds the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>     protocol feature. At this point, if negotiated, the feature only allows 
> the
>     backend to limit the number of max ram slots to a number less than
>     VHOST_MEMORY_MAX_NREGIONS = 8.
>
> 4. Transmit vhost-user memory regions individually
>     With this change, if the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>     protocol feature is enabled, Qemu will send regions to the backend using
>     individual VHOST_USER_ADD_MEM_REG and VHOST_USER_REM_MEM_REG
>     messages.
>     The max number of ram slots supported is still limited to 8.
>
> 5. Lift max memory slots imposed by vhost-user
>     With this change, if the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>     protocol feature is enabled, the backend can support a configurable 
> number of
>     ram slots up to the maximum allowed by the target platform.
>
> 6. Refactor out libvhost-user fault generation logic
>     This cleanup moves some logic from vu_set_mem_table_exec_postcopy() to a
>     separate helper, which will be needed elsewhere.
>
> 7. Support ram slot configuration in libvhost-user
>    This change adds support for processing VHOST_USER_GET_MAX_MEMSLOTS
>     messages in libvhost-user.
>     The VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol is not yet
>     enabled in libvhost-user, so at this point this change is non-functional.
>
> 8. Support adding individual regions in libvhost-user
>     This change adds libvhost-user support for mapping in new memory regions
>     when receiving VHOST_USER_ADD_MEM_REG messages.
>     The VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol is not yet
>     enabled in libvhost-user, so at this point this change is non-functional.
>
> 9. Support individual region unmap in libvhost-user
>     This change adds libvhost-user support for unmapping removed memory 
> regions
>     when receiving VHOST_USER_REM_MEM_REG messages.
>     The VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol is not yet
>     enabled in libvhost-user, so at this point this change is non-functional.
>
> 10. Lift max ram slots limit in libvhost-user
>    This change makes libvhost-user try to negotiate the
>    VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS, and adds support for
>    backends built using libvhost-user to support hot adding memory up to the
>    32 times.
>
> The changes were tested with the vhost-user-bridge sample.
>
> Changes since V3:
>     * Fixed compiler warnings caused by using pointers to packed elements
>        (flagged by patchew building with -Waddress-of-packed-member)
>
> Changes since V2:
>     * Add support for VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>        for backends build with libvhost-user
>     * Add support for postcopy live-migration when the
>        VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS protocol feature has
>        been negotiated.
>     * Add support for backends which want to support both
>        VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS and
>        VHOST_USER_PROTOCOL_F_REPLY_ACK
>     * Change feature name from VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS
>         to VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS, and any associated
>         variable names.
>     *Log a more descriptive message if the backend lowers the max ram slots 
> limit
>        on reconnect.
>
> 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 (10):
>   Add helper to populate vhost-user message regions
>   Add vhost-user helper to get MemoryRegion data
>   Add VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
>   Transmit vhost-user memory regions individually
>   Lift max memory slots limit imposed by vhost-user
>   Refactor out libvhost-user fault generation logic
>   Support ram slot configuration in libvhost-user
>   Support adding individual regions in libvhost-user
>   Support individual region unmap in libvhost-user
>   Lift max ram slots limit in libvhost-user
>
>  contrib/libvhost-user/libvhost-user.c | 341 ++++++++++++++----
>  contrib/libvhost-user/libvhost-user.h |  24 +-
>  docs/interop/vhost-user.rst           |  44 +++
>  hw/virtio/vhost-user.c                | 638 
> ++++++++++++++++++++++++++++------
>  include/hw/virtio/vhost-user.h        |   1 +
>  5 files changed, 873 insertions(+), 175 deletions(-)
>
> --
> 1.8.3.1
>