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 <wei.w.w...@intel.com> 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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