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Re: [PATCH v4 00/10] vhost-user: Lift Max Ram Slots Limitation
From: |
Raphael Norwitz |
Subject: |
Re: [PATCH v4 00/10] vhost-user: Lift Max Ram Slots Limitation |
Date: |
Thu, 4 Jun 2020 00:11:00 -0400 |
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
>
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