linux-kernel - Re: [PATCH 0/8] Virtio-over-PCIe on non-MIC

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Date:   Fri, 18 Jan 2019 16:49:16 -0700
From:   Stephen Warren <swarren@...dotorg.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     sudeep.dutt@...el.com, ashutosh.dixit@...el.com,
        gregkh@...uxfoundation.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, Vincent Whitchurch <rabinv@...s.com>,
        "ABRAHAM, KISHON VIJAY" <kishon@...com>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        linux-ntb@...glegroups.com, Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 0/8] Virtio-over-PCIe on non-MIC

On 1/16/19 9:32 AM, Vincent Whitchurch wrote:
> The Virtio-over-PCIe framework living under drivers/misc/mic/vop implements a
> generic framework to use virtio between two Linux systems, given shared memory
> and a couple of interrupts.  It does not actually require the Intel MIC
> hardware, x86-64, or even PCIe for that matter.  This patch series makes it
> buildable on more systems and adds a loopback driver to test it without special
> hardware.
> 
> Note that I don't have access to Intel MIC hardware so some testing of the
> patchset (especially the patch "vop: Use consistent DMA") on that platform
> would be appreciated, to ensure that the series does not break anything there.

So a while ago I took a look at running virtio over PCIe. I found virtio 
basically had two parts:

1) The protocol used to enumerate which virtio devices exist, and 
perhaps configure them.

2) The ring buffer protocol that actually transfers the data.

I recall that data transfer was purely based on simple shared memory and 
interrupts, and hence could run over PCIe (e.g. via the PCIe endpoint 
subsystem in the kernel) without issue.

However, the enumeration/configuration protocol requires the host to be 
able to do all kinds of strange things that can't possibly be emulated 
over PCIe; IIRC the configuration data contains "registers" that when 
written select the data other "registers" access. When the virtio device 
is exposed by a hypervisor, and all the accesses are emulated 
synchronously through a trap, this is easy enough to implement. However, 
if the two ends of this configuration parsing are on different ends of a 
PCIe bus, there's no way this can work.

Are you thinking of doing something different for 
enumeration/configuration, and just using the virtio ring buffer 
protocol over PCIe?

I did post asking about this quite a while back, but IIRC I didn't 
receive much of a response. Yes, here it is:

> https://lists.linuxfoundation.org/pipermail/virtualization/2018-March/037276.html
"virtio over SW-defined/CPU-driven PCIe endpoint"