netdev - Re: XDP offload to hypervisor

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Date:   Tue, 24 Jan 2017 04:47:46 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     John Fastabend <john.fastabend@...il.com>, jasowang@...hat.com,
        john.r.fastabend@...el.com, netdev@...r.kernel.org,
        daniel@...earbox.net
Subject: Re: XDP offload to hypervisor

On Mon, Jan 23, 2017 at 05:02:02PM -0800, Alexei Starovoitov wrote:
> > Another issue is around host/guest ABI. Guest BPF could add new features
> > at any point. What if hypervisor can not support it all?  I guess we
> > could try loading program into hypervisor and run it within guest on
> > failure to load, but this ignores question of cross-version
> > compatibility - someone might start guest on a new host
> > then try to move to an old one. So we will need an option
> > "behave like an older host" such that guest can start and then
> > move to an older host later. This will likely mean
> > implementing this validation of programs in qemu userspace unless linux
> > can supply something like this. Is this (disabling some features)
> > something that might be of interest to larger bpf community?
> 
> In case of x86->nic offload not all xdp features will be supported
> by the nic and that is expected. The user will request 'offload of xdp prog'
> in some form and if it cannot be done, then xdp programs will run
> on x86 as before. Same thing, I imagine, is applicable to virtio->host
> offload. Therefore I don't see a need for user space visible
> feature negotiation.

Not userspace visible - guest visible. As guests move between hosts,
you need to make sure source host does not commit to a feature that
destination host does not support.

-- 
MST