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Re: [Qemu-devel] [PATCH RFC] hw/pvrdma: Proposal of a new pvrdma device

From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH RFC] hw/pvrdma: Proposal of a new pvrdma device
Date: Fri, 31 Mar 2017 18:50:53 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 03/31/2017 02:38 AM, Adit Ranadive wrote:
On Thu Mar 30 2017 13:28:21 GMT-0700 (PDT), Doug Ledford wrote:
On 3/30/17 9:13 AM, Leon Romanovsky wrote:
On Thu, Mar 30, 2017 at 02:12:21PM +0300, Marcel Apfelbaum wrote:
From: Yuval Shaia <address@hidden>


 General description
 This is a very early RFC of a new RoCE emulated device
 that enables guests to use the RDMA stack without having
 a real hardware in the host.

 The current implementation supports only VM to VM communication
 on the same host.
 Down the road we plan to make possible to be able to support
 inter-machine communication by utilizing physical RoCE devices
 or Soft RoCE.

 The goals are:
 - Reach fast and secure loos-less Inter-VM data exchange.
 - Support remote VMs or bare metal machines.
 - Allow VMs migration.
 - Do not require to pin all VM memory.

 Have a QEMU implementation of the PVRDMA device. We aim to do so without
 any change in the PVRDMA guest driver which is already merged into the
 upstream kernel.

 RFC status
 The project is in early development stages and supports
 only basic send/receive operations.

 We present it so we can get feedbacks on design,
 feature demands and to receive comments from the
 community pointing us to the "right" direction.

If to judge by the feedback which you got from RDMA community
for kernel proposal [1], this community failed to understand:
1. Why do you need new module?

In this case, this is a qemu module to allow qemu to provide a virt rdma device 
to guests that is compatible with the device provided by VMWare's ESX product.  
Right now, the vmware_pvrdma driver works only when the guest is running on a 
VMWare ESX server product, this would change that.  Marcel mentioned that they 
are currently making it compatible because that's the easiest/quickest thing to 
do, but in the future they might extend beyond what VMWare's virt rdma driver 
provides/uses and might then need to either modify it to work with their 
extensions or fork and create their own virt client driver.

2. Why existing solutions are not enough and can't be extended?

This patch is against the qemu source code, not the kernel.  There is no other 
solution in the qemu source code, so there is no existing solution to extend.

3. Why RXE (SoftRoCE) can't be extended to perform this inter-VM
   communication via virtual NIC?

Eventually they want this to work on real hardware, and to be more or less 
transparent to the guest.  They will need to make it independent of the kernel 
hardware/driver in use.  That means their own virt driver, then the virt driver 
will eventually hook into whatever hardware is present on the system, or 
failing that, fall back to soft RoCE or soft iWARP if that ever makes it in the 

Hi Adit,

Hmm, this looks quite interesting.


Though I'm not surprised, the PVRDMA
device spec is relatively straightforward.

Indeed, the pvrdma driver is clear and well documented,
which made our development much easier.

I would have definitely mentioned this (if I knew about it) during my
OFA workshop talk a couple of days ago :).

There is always a next OFA workshop :)

Marcel & Yval

Doug's right. I mean basically, this looks like a QEMU version of our PVRDMA


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