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Re: [Qemu-devel] [RFC 0/3] Extend vhost-user to support VFIO based accel

From: Liang, Cunming
Subject: Re: [Qemu-devel] [RFC 0/3] Extend vhost-user to support VFIO based accelerators
Date: Tue, 2 Jan 2018 05:49:09 +0000

> -----Original Message-----
> From: Alexey Kardashevskiy [mailto:address@hidden
> Sent: Tuesday, January 2, 2018 10:42 AM
> To: Bie, Tiwei <address@hidden>; address@hidden; qemu-
> address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden
> Cc: Tan, Jianfeng <address@hidden>; Liang, Cunming
> <address@hidden>; Wang, Xiao W <address@hidden>; Wang,
> Zhihong <address@hidden>; Daly, Dan <address@hidden>
> 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.
The sample takes virtio-net device in VM as data path accelerator of virtio-net 
in nested VM.
When taking physical device on bare metal, it accelerates virtio-net in VM 
There's no additional params of QEMU command line needed for vhost-user.

One more context, including vDPA enabling in DPDK vhost-user library.

> --
> Alexey

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