Re: Out-of-Process Device Emulation session at KVM Forum 2020

[Jason Wang]

On 2020/10/30 下午2:21, Stefan Hajnoczi wrote:
> On Fri, Oct 30, 2020 at 3:04 AM Alex Williamson
> <alex.williamson@redhat.com> wrote:
> > It's great to revisit ideas, but proclaiming a uAPI is bad solely
> > because the data transfer is opaque, without defining why that's bad,
> > evaluating the feasibility and implementation of defining a well
> > specified data format rather than protocol, including cross-vendor
> > support, or proposing any sort of alternative is not so helpful imo.
> The migration approaches in VFIO and vDPA/vhost were designed for
> different requirements and I think this is why there are different
> perspectives on this. Here is a comparison and how VFIO could be
> extended in the future. I see 3 levels of device state compatibility:
>
> 1. The device cannot save/load state blobs, instead userspace fetches
> and restores specific values of the device's runtime state (e.g. last
> processed ring index). This is the vhost approach.
>
> 2. The device can save/load state in a standard format. This is
> similar to #1 except that there is a single read/write blob interface
> instead of fine-grained get_FOO()/set_FOO() interfaces. This approach
> pushes the migration state parsing into the device so that userspace
> doesn't need knowledge of every device type. With this approach it is
> possible for a device from vendor A to migrate to a device from vendor
> B, as long as they both implement the same standard migration format.
> The limitation of this approach is that vendor-specific state cannot
> be transferred.
>
> 3. The device can save/load opaque blobs. This is the initial VFIO
> approach.


I still don't get why it must be opaque.


>   A device from vendor A cannot migrate to a device from
> vendor B because the format is incompatible. This approach works well
> when devices have unique guest-visible hardware interfaces so the
> guest wouldn't be able to handle migrating a device from vendor A to a
> device from vendor B anyway.


For VFIO I guess cross vendor live migration can't succeed unless we do 
some cheats in device/vendor id.


> I think we will see more NVMe and VIRTIO hardware VFIO devices in the
> future. Those are standard guest-visible hardware interfaces. It makes
> sense to define standard migration formats so it's possible to migrate
> a device from vendor A to a device from vendor B.


Yes.


> This can be achieved as follows:
> 1. The VFIO migration blob starts with a unique format identifier such
> as a UUID. This way the destination device can identify standard
> device state formats and parse them.
> 2. The VFIO device state ioctl is extended so userspace can enumerate
> and select device state formats. This way it's possible to check
> available formats on the source and destination devices before
> migration and to configure the source device to produce device state
> in a common format.
>
> To me it seems #3 makes sense as an initial approach for VFIO since
> guest-visible hardware interfaces are often not compatible between PCI
> devices. #2 can be added in the future, especially when VFIO drivers
> from different vendors become available that present the same
> guest-visible hardware interface (NVMe, VIRTIO, etc).


For at least virtio, they will still go with virtio/vDPA. The advantages 
are:

1) virtio/vDPA can serve kernel subsystems which VFIO can't, this is 
very important for containers
2) virtio/vDPA is bus independent, we can present a virtio-mmio device 
which is based on vDPA PCI hardware for e.g microvm

I'm not familiar with NVME but they should go with the same way instead 
of depending on VFIO.

Thanks


> Stefan