Re: [Qemu-devel] [virtio-comment] [PATCH] *** Vhost-pci RFC v2 ***

[Stefan Hajnoczi]

On Mon, Sep 05, 2016 at 08:56:14AM +0000, Marc-André Lureau wrote:
> Hi
> 
> On Sat, Sep 3, 2016 at 5:36 PM Wang, Wei W <address@hidden> wrote:
> 
> > Marc-André and I just got different thoughts about a design direction. I
> > prefer to have all the frontend virtio devices (net, scsi, console etc.)
> > from the same VM to be supported by one backend vhost-pci device (N-1),
> > while Marc-André prefers to have each frontend virtio device be supported
> > by a backend vhost-pci device (N-N).
> >
> 
> I suggested 1-1 (not n-n, but you can have several 1-1 pairs), unless you
> have a good reason to do differently, starting from the use case (is there
> a case that requires several backends/consumers in the same VM? if yes, 1-1
> design could still fit). If it's to save guest memory space, it may not be
> a good enough reason, but I don't see clearly the implications.

N-1 saves address space but is probably a poor fit for modern PCI
devices that are geared towards IOMMUs.

Each virtio device should be isolated in terms of memory space and
hotplug/reset life cycle.  This ensures they are robust against driver
bugs and can be safely delegated/passed through to different
applications or nested VMs that don't trust each other.

Isolation between virtio device instances sharing a single vhost-pci
device (N-1) will harder to achieve.  I would aim for the simpler 1-1
design instead where each device is isolated.

Are there specific reasons for wanting an N-1 design?

Stefan

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