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

[Doug Ledford]

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>
> >
> >  Hi,
> >
> >  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.
> >
> >
> >  Objective
> >  =========
> >  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 kernel.


> Can you please help us to fill this knowledge gap?
>
> [1] http://marc.info/?l=linux-rdma&m=149063626907175&w=2