linux-kernel - Re: [PATCH V3 2/4] misc: vop: do not allocate and reassign the used ring

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Date:   Thu, 29 Oct 2020 06:35:11 -0700
From:   "Dixit, Ashutosh" <ashutosh.dixit@...el.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Sherry Sun <sherry.sun@....com>,
        "Dutt, Sudeep" <sudeep.dutt@...el.com>,
        dl-linux-imx <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "kishon@...com" <kishon@...com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3 2/4] misc: vop: do not allocate and reassign the used ring

On Thu, 29 Oct 2020 04:53:09 -0700, Arnd Bergmann wrote:
>
> On Thu, Oct 29, 2020 at 11:07 AM Vincent Whitchurch
> <vincent.whitchurch@...s.com> wrote:
> >
> > On Wed, Oct 28, 2020 at 04:50:36PM +0100, Arnd Bergmann wrote:
> > > I think we should try to do something on top of the PCIe endpoint subsystem
> > > to make it work across arbitrary combinations of host and device
> > > implementations,
> > > and provide a superset of what the MIC driver, (out-of-tree) Bluefield endpoint
> > > driver, and the NTB subsystem as well as a couple of others used to do,
> > > each of them tunneling block/network/serial/... over a PCIe link of some
> > > sort, usually with virtio.
> >
> > VOP is not PCIe-specific (as demonstrated by the vop-loopback patches I
> > posted a while ago [1]), and it would be a shame for a replacement to be
> > tied to the PCIe endpoint subsystem.  There are many SOCs out there
> > which have multiple Linux-capable processors without cache-coherency
> > between them.  VOP is (or should I say was since I guess it's being
> > deleted) the closest we have in mainline to easily get generic virtio
> > (and not just rpmsg) running between these kind of Linux instances.  If
> > a new replacement framework were to be PCIe-exclusive then we'd have to
> > invent one more framework for non-PCIe links to do pretty much the same
> > thing.
> >
> > [1] https://lore.kernel.org/lkml/20190403104746.16063-1-vincent.whitchurch@axis.com/
>
> Right, sorry I forgot about that. I think this means we should keep having
> an abstraction between VOP (under whichever name) and the lower levels,
> and be aware that it might run on any number of these:
>
> - PCIe endpoint, with the endpoint controlling the virtio configuration
> - PCIe endpoint, with the host (the side that has the pci_driver) controlling
>   the virtio configuration
> - NTB connections
> - your  loopback mode
> - Virtio tunnels between VM guests (see https://www.linaro.org/projects/#STR)
> - Intel MIC (to be removed, but it would be wrong to make assumptions that
>   cannot be made on that type of hardware)

A virtio interface being one between host and guest is inherently
asymmetric. The whole innovation of the VOP design was to treat Linux on a
PCIe device as a guest, there was even talk at some point of the "guest"
being managed via libvirt. So here host and guest retain their specific
role/personality. The host "inserts" devices which appear in the guest
e.g. So I am not sure how this asymmetry plays in the scenarios mentioned
above.