Re: [Qemu-devel] [virtio-dev] [PATCH v1 6/6] vhost-user: add VFIO based accelerators support

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/01/2018 05:03, Tiwei Bie wrote:
> The key difference with PCI passthru is that, in this case only
> the data path of the device (e.g. DMA ring, notify region and
> queue interrupt) is pass-throughed to the VM, the device control
> path (e.g. PCI configuration space and MMIO regions) is still
> defined and emulated by QEMU.
> 
> The benefits of keeping virtio device emulation in QEMU compared
> with virtio device PCI passthru include (but not limit to):
> 
> - consistent device interface for guest OS in the VM;
> - max flexibility on the hardware (i.e. the accelerators) design;
> - leveraging the existing virtio live-migration framework;
> 
> The virtual IOMMU isn't supported by the accelerators for now.
> Because vhost-user currently lacks of an efficient way to share
> the IOMMU table in VM to vhost backend. That's why the software
> implementation of virtual IOMMU support in vhost-user backend
> can't support dynamic mapping well. Once this problem is solved
> in vhost-user, virtual IOMMU can be supported by accelerators
> too, and the IOMMU feature bit checking in this patch can be
> removed.

I don't understand why this would use vhost-user.  vhost-user is meant
for connecting to e.g. a user-space switch that is shared between
multiple virtual machines.

In this case, there would be one VFIO device per VM (because different
VM must be in different VFIO groups).  So I don't understand the benefit
of configuring the control path of the VFIO device outside QEMU.

Paolo