RE: [Qemu-devel] [PATCH v1 0/6] Extend vhost-user to support VFIO based accelerators

["Liang, Cunming" <cunming.liang@intel.com>]

> -----Original Message-----
> From: Stefan Hajnoczi [mailto:stefanha@gmail.com]
> Sent: Thursday, January 25, 2018 10:22 PM
> To: Bie, Tiwei <tiwei.bie@intel.com>
> Cc: qemu-devel@nongnu.org; virtio-dev@lists.oasis-open.org; 
> mst@redhat.com; alex.williamson@redhat.com; jasowang@redhat.com; 
> pbonzini@redhat.com; stefanha@redhat.com; Tan, Jianfeng 
> <jianfeng.tan@intel.com>; Liang, Cunming <cunming.liang@intel.com>; 
> Wang, Xiao W <xiao.w.wang@intel.com>; Wang, Zhihong 
> <zhihong.wang@intel.com>; Daly, Dan <dan.daly@intel.com>
> Subject: Re: [Qemu-devel] [PATCH v1 0/6] Extend vhost-user to support 
> VFIO based accelerators
> 
> On Thu, Jan 25, 2018 at 12:03:22PM +0800, Tiwei Bie wrote:
> > 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.
> >
> > The previous links:
> > RFC:
> > http://lists.nongnu.org/archive/html/qemu-devel/2017-12/msg04844.htm
> > l
> 
> Is vDPA also useful in the simpler use case where there is no NFVi application?
There're separate patches to have non-DPDK vDPA(aka. vhost-vfio, a new vhost backend proposed) support, which is the scope of stage-II as we mentioned in community call.
Stay tuned for the RFC patch. Main idea is that the virtio compatible device driver register as a mdev to talk with qemu vhost-vfio. 

> 
> In other words, you have virtio-net inside the guest and vhost-net on the host.
> You'd like to accelerate the virtio-net NIC using the hardware's vDPA support.
> 
> DPDK requires dedicated logical cores and hugepages.  Those resources 
> will not be used (wasted) if you just want to enable vDPA but have no 
> DPDK packet processing application.
> 
> How can this use case be supported without wasting resources?
> 
> Stefan