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Subject: Re: [virtio-dev] [RFC 0/3] Extend vhost-user to support VFIO based accelerators
On 2017年12月22日 14:41, Tiwei Bie wrote:
This RFC 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 passthru while keeping the virtio device emulation in QEMU. When we have virtio ring compatible devices, it's possible to setup the device (DMA mapping, PCI config, etc) based on the existing info (memory-table, features, vring info, etc) which is available on the vhost-backend (e.g. DPDK vhost library). Then, we will be able to use such devices to accelerate the emulated device for the VM. And we call it vDPA: vhost DataPath Acceleration. The key difference between VFIO passthru and vDPA is that, in vDPA only the data path (e.g. ring, notify and queue interrupt) is pass-throughed, 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 VFIO passthru include (but not limit to): - consistent device interface from guest OS; - max flexibility on control path and hardware design; - leveraging the existing virtio live-migration framework; But 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 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 VFIO passthru to achieve the data path pass through. A new protocol feature bit is added to negotiate the accelerator feature support. Two new slave message types are added to enable the notify and 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. There are some rough edges in this patch set (so this is a RFC patch set for now), but it's never too early to hear the thoughts from the community! So any comments and suggestions would be really appreciated! Tiwei Bie (3): vhost-user: support receiving file descriptors in slave_read vhost-user: introduce shared vhost-user state vhost-user: add VFIO based accelerators support docs/interop/vhost-user.txt | 57 ++++++ hw/scsi/vhost-user-scsi.c | 6 +- hw/vfio/common.c | 2 +- hw/virtio/vhost-user.c | 430 +++++++++++++++++++++++++++++++++++++++- hw/virtio/vhost.c | 3 +- hw/virtio/virtio-pci.c | 8 - hw/virtio/virtio-pci.h | 8 + include/hw/vfio/vfio.h | 2 + include/hw/virtio/vhost-user.h | 43 ++++ include/hw/virtio/virtio-scsi.h | 6 +- net/vhost-user.c | 30 +-- 11 files changed, 561 insertions(+), 34 deletions(-) create mode 100644 include/hw/virtio/vhost-user.h
I may miss something, but may I ask why you must implement them through vhost-use/dpdk. It looks to me you could put all of them in qemu which could simplify a lots of things (just like userspace NVME driver wrote by Fam).