Re: [Qemu-devel] [PATCH v2 0/6] Extend vhost-user to support VFIO based accelerators

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

On Mon, Mar 19, 2018 at 03:15:31PM +0800, Tiwei Bie wrote:
> This patch set does some small extensions to vhost-user protocol
> to support VFIO based accelerators, and makes it possible to get
> the similar performance of VFIO based PCI passthru while keeping
> the virtio device emulation in QEMU.

I love your patches!
Yet there are some things to improve.
Posting comments separately as individual messages.


> How does accelerator accelerate vhost (data path)
> =================================================
> 
> Any virtio ring compatible devices potentially can be used as the
> vhost data path accelerators. We can setup the accelerator based
> on the informations (e.g. memory table, features, ring info, etc)
> available on the vhost backend. And accelerator will be able to use
> the virtio ring provided by the virtio driver in the VM directly.
> So the virtio driver in the VM can exchange e.g. network packets
> with the accelerator directly via the virtio ring. That is to say,
> we will be able to use the accelerator to accelerate the vhost
> data path. We call it vDPA: vhost Data Path Acceleration.
> 
> Notice: Although the accelerator can talk with the virtio driver
> in the VM via the virtio ring directly. The control path events
> (e.g. device start/stop) in the VM will still be trapped and handled
> by QEMU, and QEMU will deliver such events to the vhost backend
> via standard vhost protocol.
> 
> Below link is an example showing how to setup a such environment
> via nested VM. In this case, the virtio device in the outer VM is
> the accelerator. It will be used to accelerate the virtio device
> in the inner VM. In reality, we could use virtio ring compatible
> hardware device as the accelerators.
> 
> http://dpdk.org/ml/archives/dev/2017-December/085044.html
> 
> In above example, it doesn't require any changes to QEMU, but
> it has lower performance compared with the traditional VFIO
> based PCI passthru. And that's the problem this patch set wants
> to solve.
> 
> The performance issue of vDPA/vhost-user and solutions
> ======================================================
> 
> For vhost-user backend, the critical issue in vDPA is that the
> data path performance is relatively low and some host threads are
> needed for the data path, because some necessary mechanisms are
> missing to support:
> 
> 1) guest driver notifies the device directly;
> 2) device interrupts the guest directly;
> 
> So this patch set does some small extensions to the vhost-user
> protocol to make both of them possible. It leverages the same
> mechanisms (e.g. EPT and Posted-Interrupt on Intel platform) as
> the PCI passthru.
> 
> A new protocol feature bit is added to negotiate the accelerator
> feature support. Two new slave message types are added to control
> the notify region and queue interrupt passthru for each queue.
> >From the view of vhost-user protocol design, it's very flexible.
> The passthru can be enabled/disabled for each queue individually,
> and it's possible to accelerate each queue by different devices.
> More design and implementation details can be found from the last
> patch.
> 
> Difference between vDPA and PCI passthru
> ========================================
> 
> The key difference between PCI passthru and vDPA is that, in vDPA
> only the data path of the device (e.g. DMA ring, notify region and
> queue interrupt) is pass-throughed to the VM, the device control
> path (e.g. PCI configuration space and MMIO regions) is still
> defined and emulated by QEMU.
> 
> The benefits of keeping virtio device emulation in QEMU compared
> with virtio device PCI passthru include (but not limit to):
> 
> - consistent device interface for guest OS in the VM;
> - max flexibility on the hardware (i.e. the accelerators) design;
> - leveraging the existing virtio live-migration framework;
> 
> Why extend vhost-user for vDPA
> ==============================
> 
> We have already implemented various virtual switches (e.g. OVS-DPDK)
> based on vhost-user for VMs in the Cloud. They are purely software
> running on CPU cores. When we have accelerators for such NFVi applications,
> it's ideal if the applications could keep using the original interface
> (i.e. vhost-user netdev) with QEMU, and infrastructure is able to decide
> when and how to switch between CPU and accelerators within the interface.
> And the switching (i.e. switch between CPU and accelerators) can be done
> flexibly and quickly inside the applications.
> 
> More details about this can be found from the Cunming's discussions on
> the RFC patch set.
> 
> Update notes
> ============
> 
> IOMMU feature bit check is removed in this version, because:
> 
> The IOMMU feature is negotiable, when an accelerator is used and
> it doesn't support virtual IOMMU, its driver just won't provide
> this feature bit when vhost library querying its features. And if
> it supports the virtual IOMMU, its driver can provide this feature
> bit. It's not reasonable to add this limitation in this patch set.
> 
> The previous links:
> RFC: http://lists.nongnu.org/archive/html/qemu-devel/2017-12/msg04844.html
> v1:  http://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg06028.html
> 
> v1 -> v2:
> - Add some explanations about why extend vhost-user in commit log (Paolo);
> - Bug fix in slave_read() according to Stefan's fix in DPDK;
> - Remove IOMMU feature check and related commit log;
> - Some minor refinements;
> - Rebase to the latest QEMU;
> 
> RFC -> v1:
> - Add some details about how vDPA works in cover letter (Alexey)
> - Add some details about the OVS offload use-case in cover letter (Jason)
> - Move PCI specific stuffs out of vhost-user (Jason)
> - Handle the virtual IOMMU case (Jason)
> - Move VFIO group management code into vfio/common.c (Alex)
> - Various refinements;
> (approximately sorted by comment posting time)
> 
> Tiwei Bie (6):
>   vhost-user: support receiving file descriptors in slave_read
>   vhost-user: introduce shared vhost-user state
>   virtio: support adding sub-regions for notify region
>   vfio: support getting VFIOGroup from groupfd
>   vfio: remove DPRINTF() definition from vfio-common.h
>   vhost-user: add VFIO based accelerators support
> 
>  Makefile.target                 |   4 +
>  docs/interop/vhost-user.txt     |  57 +++++++++
>  hw/scsi/vhost-user-scsi.c       |   6 +-
>  hw/vfio/common.c                |  97 +++++++++++++++-
>  hw/virtio/vhost-user.c          | 248 +++++++++++++++++++++++++++++++++++++++-
>  hw/virtio/virtio-pci.c          |  48 ++++++++
>  hw/virtio/virtio-pci.h          |   5 +
>  hw/virtio/virtio.c              |  39 +++++++
>  include/hw/vfio/vfio-common.h   |  11 +-
>  include/hw/virtio/vhost-user.h  |  34 ++++++
>  include/hw/virtio/virtio-scsi.h |   6 +-
>  include/hw/virtio/virtio.h      |   5 +
>  include/qemu/osdep.h            |   1 +
>  net/vhost-user.c                |  30 ++---
>  scripts/create_config           |   3 +
>  15 files changed, 561 insertions(+), 33 deletions(-)
>  create mode 100644 include/hw/virtio/vhost-user.h
> 
> -- 
> 2.11.0