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    SubjectRe: [RFC][PATCH v3 1/3] A device for zero-copy based on KVM virtio-net.
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    On Wednesday 14 April 2010 22:31:42 Michael S. Tsirkin wrote:
    > On Wed, Apr 14, 2010 at 06:35:57PM +0200, Arnd Bergmann wrote:
    > > On Wednesday 14 April 2010, Michael S. Tsirkin wrote:
    > > > > >
    > > > > > qemu needs the ability to inject raw packets into device
    > > > > > from userspace, bypassing vhost/virtio (for live migration).
    > > > >
    > > > > Ok, but since there is only a write callback and no read, it won't
    > > > > actually be able to do this with the current code, right?
    > > >
    > > > I think it'll work as is, with vhost qemu only ever writes,
    > > > never reads from device. We'll also never need GSO etc
    > > > which is a large part of what tap does (and macvtap will
    > > > have to do).
    > >
    > > Ah, I see. I didn't realize that qemu needs to write to the
    > > device even if vhost is used. But for the case of migration to
    > > another machine without vhost, wouldn't qemu also need to read?
    >
    > Not that I know. Why?

    Well, if the guest not only wants to send data but also receive
    frames coming from other machines, they need to get from the kernel
    into qemu, and the only way I can see for doing that is to read
    from this device if there is no vhost support around on the new
    machine.

    Maybe we're talking about different things here.

    Arnd


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