linux-kernel - Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

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Message-ID: <1493101803.3171.246.camel@oracle.com>
Date:   Tue, 25 Apr 2017 08:30:03 +0200
From:   Knut Omang <knut.omang@...cle.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steve Wise <swise@...ngridcomputing.com>,
        Stephen Bates <sbates@...thlin.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>
Subject: Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

On Mon, 2017-04-24 at 10:14 -0600, Logan Gunthorpe wrote:
> 
> On 24/04/17 01:36 AM, Knut Omang wrote:
> > My first reflex when reading this thread was to think that this whole domain
> > lends it self excellently to testing via Qemu. Could it be that doing this in 
> > the opposite direction might be a safer approach in the long run even though 
> > (significant) more work up-front?
> 
> That's an interesting idea. We did do some very limited testing on qemu
> with one iteration of our work. However, it's difficult because there is
> no support for any RDMA devices which are a part of our primary use
> case. 

Yes, that's why I used 'significant'. One good thing is that given resources 
it can easily be done in parallel with other development, and will give additional
insight of some form.

> I also imagine it would be quite difficult to develop those models
> given the array of hardware that needs to be supported and the deep
> functional knowledge required to figure out appropriate restrictions.

>From my naive perspective it seems it need not even be a full model to get some benefits,
just low level functionality tests with some instances of a
device that offers some MMIO space 'playground'.

Or maybe you can leverage some of the already implemented emulated devices in Qemu?

Knut

> 
> Logan