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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [RFC 0/3] Extend vhost-user to support VFIO based accelerators
Date: Tue, 2 Jan 2018 13:42:08 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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.


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