Re: [PATCH v5 00/15] vDPA shadow virtqueue

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 08, 2022 at 03:14:35PM +0800, Jason Wang wrote:
> On Tue, Mar 8, 2022 at 3:11 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Mar 08, 2022 at 02:03:32PM +0800, Jason Wang wrote:
> > >
> > > 在 2022/3/7 下午11:33, Eugenio Pérez 写道:
> > > > This series enable shadow virtqueue (SVQ) for vhost-vdpa devices. This
> > > > is intended as a new method of tracking the memory the devices touch
> > > > during a migration process: Instead of relay on vhost device's dirty
> > > > logging capability, SVQ intercepts the VQ dataplane forwarding the
> > > > descriptors between VM and device. This way qemu is the effective
> > > > writer of guests memory, like in qemu's virtio device operation.
> > > >
> > > > When SVQ is enabled qemu offers a new virtual address space to the
> > > > device to read and write into, and it maps new vrings and the guest
> > > > memory in it. SVQ also intercepts kicks and calls between the device
> > > > and the guest. Used buffers relay would cause dirty memory being
> > > > tracked.
> > > >
> > > > This effectively means that vDPA device passthrough is intercepted by
> > > > qemu. While SVQ should only be enabled at migration time, the switching
> > > > from regular mode to SVQ mode is left for a future series.
> > > >
> > > > It is based on the ideas of DPDK SW assisted LM, in the series of
> > > > DPDK's https://patchwork.dpdk.org/cover/48370/ . However, these does
> > > > not map the shadow vq in guest's VA, but in qemu's.
> > > >
> > > > For qemu to use shadow virtqueues the guest virtio driver must not use
> > > > features like event_idx.
> > > >
> > > > SVQ needs to be enabled with cmdline:
> > > >
> > > > -netdev type=vhost-vdpa,vhostdev=vhost-vdpa-0,id=vhost-vdpa0,svq=on
> >
> > A stable API for an incomplete feature is a problem imho.
> 
> It should be "x-svq".


Well look at patch 15.

> >
> >
> > > >
> > > > The first three patches enables notifications forwarding with
> > > > assistance of qemu. It's easy to enable only this if the relevant
> > > > cmdline part of the last patch is applied on top of these.
> > > >
> > > > Next four patches implement the actual buffer forwarding. However,
> > > > address are not translated from HVA so they will need a host device with
> > > > an iommu allowing them to access all of the HVA range.
> > > >
> > > > The last part of the series uses properly the host iommu, so qemu
> > > > creates a new iova address space in the device's range and translates
> > > > the buffers in it. Finally, it adds the cmdline parameter.
> > > >
> > > > Some simple performance tests with netperf were done. They used a nested
> > > > guest with vp_vdpa, vhost-kernel at L0 host. Starting with no svq and a
> > > > baseline average of ~9009.96Mbps:
> > > > Recv   Send    Send
> > > > Socket Socket  Message  Elapsed
> > > > Size   Size    Size     Time     Throughput
> > > > bytes  bytes   bytes    secs.    10^6bits/sec
> > > > 131072  16384  16384    30.01    9061.03
> > > > 131072  16384  16384    30.01    8962.94
> > > > 131072  16384  16384    30.01    9005.92
> > > >
> > > > To enable SVQ buffers forwarding reduce throughput to about
> > > > Recv   Send    Send
> > > > Socket Socket  Message  Elapsed
> > > > Size   Size    Size     Time     Throughput
> > > > bytes  bytes   bytes    secs.    10^6bits/sec
> > > > 131072  16384  16384    30.01    7689.72
> > > > 131072  16384  16384    30.00    7752.07
> > > > 131072  16384  16384    30.01    7750.30
> > > >
> > > > However, many performance improvements were left out of this series for
> > > > simplicity, so difference should shrink in the future.
> > > >
> > > > Comments are welcome.
> > >
> > >
> > > Hi Michael:
> > >
> > > What do you think of this series? It looks good to me as a start. The
> > > feature could only be enabled as a dedicated parameter. If you're ok, I'd
> > > try to make it for 7.0.
> > >
> > > Thanks
> >
> > Well that's cutting it awfully close, and it's not really useful
> > at the current stage, is it?
> 
> This allows vDPA to be migrated when using "x-svq=on".
> But anyhow it's
> experimental.

it's less experimental than incomplete. It seems pretty clearly not
the way it will work down the road, we don't want svq involved
at all times.

> >
> > The IOVA trick does not feel complete either.
> 
> I don't get here. We don't use any IOVA trick as DPDK (it reserve IOVA
> for shadow vq) did. So we won't suffer from the issues of DPDK.
> 
> Thanks

Maybe I misundrstand how this all works.
I refer to all the iova_tree_alloc_map things.

> >
> > >
> > > >
> > > > TODO on future series:
> > > > * Event, indirect, packed, and others features of virtio.
> > > > * To support different set of features between the device<->SVQ and the
> > > >    SVQ<->guest communication.
> > > > * Support of device host notifier memory regions.
> > > > * To sepparate buffers forwarding in its own AIO context, so we can
> > > >    throw more threads to that task and we don't need to stop the main
> > > >    event loop.
> > > > * Support multiqueue virtio-net vdpa.
> > > > * Proper documentation.
> > > >
> > > > Changes from v4:
> > > > * Iterate iova->hva tree instead on maintain own tree so we support HVA
> > > >    overlaps.
> > > > * Fix: Errno completion at failure.
> > > > * Rename x-svq to svq, so changes to stable does not affect cmdline parameter.
> > > >
> > > > Changes from v3:
> > > > * Add @unstable feature to NetdevVhostVDPAOptions.x-svq.
> > > > * Fix uncomplete mapping (by 1 byte) of memory regions if svq is enabled.
> > > > v3 link:
> > > > https://lore.kernel.org/qemu-devel/20220302203012.3476835-1-eperezma@redhat.com/
> > > >
> > > > Changes from v2:
> > > > * Less assertions and more error handling in iova tree code.
> > > > * Better documentation, both fixing errors and making @param: format
> > > > * Homogeneize SVQ avail_idx_shadow and shadow_used_idx to make shadow a
> > > >    prefix at both times.
> > > > * Fix: Fo not use VirtQueueElement->len field, track separatedly.
> > > > * Split vhost_svq_{enable,disable}_notification, so the code looks more
> > > >    like the kernel driver code.
> > > > * Small improvements.
> > > > v2 link:
> > > > https://lore.kernel.org/all/CAJaqyWfXHE0C54R_-OiwJzjC0gPpkE3eX0L8BeeZXGm1ERYPtA@mail.gmail.com/
> > > >
> > > > Changes from v1:
> > > > * Feature set at device->SVQ is now the same as SVQ->guest.
> > > > * Size of SVQ is not max available device size anymore, but guest's
> > > >    negotiated.
> > > > * Add VHOST_FILE_UNBIND kick and call fd treatment.
> > > > * Make SVQ a public struct
> > > > * Come back to previous approach to iova-tree
> > > > * Some assertions are now fail paths. Some errors are now log_guest.
> > > > * Only mask _F_LOG feature at vdpa_set_features svq enable path.
> > > > * Refactor some errors and messages. Add missing error unwindings.
> > > > * Add memory barrier at _F_NO_NOTIFY set.
> > > > * Stop checking for features flags out of transport range.
> > > > v1 link:
> > > > https://lore.kernel.org/virtualization/7d86c715-6d71-8a27-91f5-8d47b71e3201@redhat.com/
> > > >
> > > > Changes from v4 RFC:
> > > > * Support of allocating / freeing iova ranges in IOVA tree. Extending
> > > >    already present iova-tree for that.
> > > > * Proper validation of guest features. Now SVQ can negotiate a
> > > >    different set of features with the device when enabled.
> > > > * Support of host notifiers memory regions
> > > > * Handling of SVQ full queue in case guest's descriptors span to
> > > >    different memory regions (qemu's VA chunks).
> > > > * Flush pending used buffers at end of SVQ operation.
> > > > * QMP command now looks by NetClientState name. Other devices will need
> > > >    to implement it's way to enable vdpa.
> > > > * Rename QMP command to set, so it looks more like a way of working
> > > > * Better use of qemu error system
> > > > * Make a few assertions proper error-handling paths.
> > > > * Add more documentation
> > > > * Less coupling of virtio / vhost, that could cause friction on changes
> > > > * Addressed many other small comments and small fixes.
> > > >
> > > > Changes from v3 RFC:
> > > >    * Move everything to vhost-vdpa backend. A big change, this allowed
> > > >      some cleanup but more code has been added in other places.
> > > >    * More use of glib utilities, especially to manage memory.
> > > > v3 link:
> > > > https://lists.nongnu.org/archive/html/qemu-devel/2021-05/msg06032.html
> > > >
> > > > Changes from v2 RFC:
> > > >    * Adding vhost-vdpa devices support
> > > >    * Fixed some memory leaks pointed by different comments
> > > > v2 link:
> > > > https://lists.nongnu.org/archive/html/qemu-devel/2021-03/msg05600.html
> > > >
> > > > Changes from v1 RFC:
> > > >    * Use QMP instead of migration to start SVQ mode.
> > > >    * Only accepting IOMMU devices, closer behavior with target devices
> > > >      (vDPA)
> > > >    * Fix invalid masking/unmasking of vhost call fd.
> > > >    * Use of proper methods for synchronization.
> > > >    * No need to modify VirtIO device code, all of the changes are
> > > >      contained in vhost code.
> > > >    * Delete superfluous code.
> > > >    * An intermediate RFC was sent with only the notifications forwarding
> > > >      changes. It can be seen in
> > > >      https://patchew.org/QEMU/20210129205415.876290-1-eperezma@redhat.com/
> > > > v1 link:
> > > > https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg05372.html
> > > >
> > > > Eugenio Pérez (20):
> > > >        virtio: Add VIRTIO_F_QUEUE_STATE
> > > >        virtio-net: Honor VIRTIO_CONFIG_S_DEVICE_STOPPED
> > > >        virtio: Add virtio_queue_is_host_notifier_enabled
> > > >        vhost: Make vhost_virtqueue_{start,stop} public
> > > >        vhost: Add x-vhost-enable-shadow-vq qmp
> > > >        vhost: Add VhostShadowVirtqueue
> > > >        vdpa: Register vdpa devices in a list
> > > >        vhost: Route guest->host notification through shadow virtqueue
> > > >        Add vhost_svq_get_svq_call_notifier
> > > >        Add vhost_svq_set_guest_call_notifier
> > > >        vdpa: Save call_fd in vhost-vdpa
> > > >        vhost-vdpa: Take into account SVQ in vhost_vdpa_set_vring_call
> > > >        vhost: Route host->guest notification through shadow virtqueue
> > > >        virtio: Add vhost_shadow_vq_get_vring_addr
> > > >        vdpa: Save host and guest features
> > > >        vhost: Add vhost_svq_valid_device_features to shadow vq
> > > >        vhost: Shadow virtqueue buffers forwarding
> > > >        vhost: Add VhostIOVATree
> > > >        vhost: Use a tree to store memory mappings
> > > >        vdpa: Add custom IOTLB translations to SVQ
> > > >
> > > > Eugenio Pérez (15):
> > > >    vhost: Add VhostShadowVirtqueue
> > > >    vhost: Add Shadow VirtQueue kick forwarding capabilities
> > > >    vhost: Add Shadow VirtQueue call forwarding capabilities
> > > >    vhost: Add vhost_svq_valid_features to shadow vq
> > > >    virtio: Add vhost_svq_get_vring_addr
> > > >    vdpa: adapt vhost_ops callbacks to svq
> > > >    vhost: Shadow virtqueue buffers forwarding
> > > >    util: Add iova_tree_alloc_map
> > > >    util: add iova_tree_find_iova
> > > >    vhost: Add VhostIOVATree
> > > >    vdpa: Add custom IOTLB translations to SVQ
> > > >    vdpa: Adapt vhost_vdpa_get_vring_base to SVQ
> > > >    vdpa: Never set log_base addr if SVQ is enabled
> > > >    vdpa: Expose VHOST_F_LOG_ALL on SVQ
> > > >    vdpa: Add x-svq to NetdevVhostVDPAOptions
> > > >
> > > >   qapi/net.json                      |   8 +-
> > > >   hw/virtio/vhost-iova-tree.h        |  27 ++
> > > >   hw/virtio/vhost-shadow-virtqueue.h |  87 ++++
> > > >   include/hw/virtio/vhost-vdpa.h     |   8 +
> > > >   include/qemu/iova-tree.h           |  38 +-
> > > >   hw/virtio/vhost-iova-tree.c        | 110 +++++
> > > >   hw/virtio/vhost-shadow-virtqueue.c | 637 +++++++++++++++++++++++++++++
> > > >   hw/virtio/vhost-vdpa.c             | 525 +++++++++++++++++++++++-
> > > >   net/vhost-vdpa.c                   |  48 ++-
> > > >   util/iova-tree.c                   | 169 ++++++++
> > > >   hw/virtio/meson.build              |   2 +-
> > > >   11 files changed, 1633 insertions(+), 26 deletions(-)
> > > >   create mode 100644 hw/virtio/vhost-iova-tree.h
> > > >   create mode 100644 hw/virtio/vhost-shadow-virtqueue.h
> > > >   create mode 100644 hw/virtio/vhost-iova-tree.c
> > > >   create mode 100644 hw/virtio/vhost-shadow-virtqueue.c
> > > >
> >