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    SubjectRe: [RFC PATCH 00/17] virtual-bus
    Herbert Xu wrote:
    > Avi Kivity <avi@redhat.com> wrote:
    >
    >> virtio is already non-kvm-specific (lguest uses it) and non-pci-specific
    >> (s390 uses it).
    >>
    >
    > I think Greg's work shows that putting the backend in the kernel
    > can dramatically reduce the cost of a single guest->host transaction.
    > I'm sure the same thing would work for virtio too.
    >

    Virtio suffers because we've had no notification of when a packet is
    actually submitted. With the notification, the only difference should
    be in the cost of a kernel->user switch, which is nowhere nearly as
    dramatic.

    >> If you have a good exit mitigation scheme you can cut exits by a factor
    >> of 100; so the userspace exit costs are cut by the same factor. If you
    >> have good copyless networking APIs you can cut the cost of copies to
    >> zero (well, to the cost of get_user_pages_fast(), but a kernel solution
    >> needs that too).
    >>
    >
    > Given the choice of having to mitigate or not having the problem
    > in the first place, guess what I would prefer :)
    >

    There is no choice. Exiting from the guest to the kernel to userspace
    is prohibitively expensive, you can't do that on every packet.

    --
    error compiling committee.c: too many arguments to function



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