> -----Original Message----- > From: Alexey Kardashevskiy [mailto:a...@ozlabs.ru] > Sent: Tuesday, January 2, 2018 2:01 PM > To: Liang, Cunming <cunming.li...@intel.com>; Bie, Tiwei > <tiwei....@intel.com>; > virtio-...@lists.oasis-open.org; qemu-devel@nongnu.org; m...@redhat.com; > alex.william...@redhat.com; pbonz...@redhat.com; stefa...@redhat.com > Cc: Tan, Jianfeng <jianfeng....@intel.com>; Wang, Xiao W > <xiao.w.w...@intel.com>; Wang, Zhihong <zhihong.w...@intel.com>; Daly, > Dan <dan.d...@intel.com> > Subject: Re: [Qemu-devel] [RFC 0/3] Extend vhost-user to support VFIO based > accelerators > > On 02/01/18 16:49, Liang, Cunming wrote: > > > > > >> -----Original Message----- > >> From: Alexey Kardashevskiy [mailto:a...@ozlabs.ru] > >> Sent: Tuesday, January 2, 2018 10:42 AM > >> To: Bie, Tiwei <tiwei....@intel.com>; > >> virtio-...@lists.oasis-open.org; qemu- de...@nongnu.org; > >> m...@redhat.com; alex.william...@redhat.com; pbonz...@redhat.com; > >> stefa...@redhat.com > >> Cc: Tan, Jianfeng <jianfeng....@intel.com>; Liang, Cunming > >> <cunming.li...@intel.com>; Wang, Xiao W <xiao.w.w...@intel.com>; > >> Wang, Zhihong <zhihong.w...@intel.com>; Daly, Dan > >> <dan.d...@intel.com> > >> Subject: Re: [Qemu-devel] [RFC 0/3] Extend vhost-user to support VFIO > >> based accelerators > >> > >> On 22/12/17 17:41, 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! > >> > >> I am missing a lot of context here. Out of curiosity - how is this > >> all supposed to work? QEMU command line example would be useful, what > >> will the guest see? A virtio device (i.e. Redhat vendor ID) or an > >> actual PCI device (since VFIO is mentioned)? Thanks. > > > > It's a normal virtio PCIe devices in the guest. Extensions on the host are > transparent to the guest. > > > > In terms of the usage, there's a sample may help. > > http://dpdk.org/ml/archives/dev/2017-December/085044.html > > The sample takes virtio-net device in VM as data path accelerator of > > virtio-net > in nested VM. > > > Aaah, this is for nested VMs, the original description was not clear about > this. I > get it now, thanks.
BTW, the patch is not only used for nested VM, even the sample is. Once you get a virtio compatible device, it's helpful to normal VM too. Basically, it gives extra ability of para-virtualized device to associate with an accelerator who can talk with the guest PV device driver directly. > > > > When taking physical device on bare metal, it accelerates virtio-net in VM > equivalently. > > There's no additional params of QEMU command line needed for vhost-user. > > > > One more context, including vDPA enabling in DPDK vhost-user library. > > http://dpdk.org/ml/archives/dev/2017-December/084792.html > > > > -- > Alexey
