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    SubjectRe: [PATCHv2] virtio: put last seen used index into ring itself
    Date
    "Michael S. Tsirkin" <mst@redhat.com> wrote:
    > Generally, the Host end of the virtio ring doesn't need to see where
    > Guest is up to in consuming the ring. However, to completely understand
    > what's going on from the outside, this information must be exposed.
    > For example, host can reduce the number of interrupts by detecting
    > that the guest is currently handling previous buffers.
    >
    > Fortunately, we have room to expand: the ring is always a whole number
    > of pages and there's hundreds of bytes of padding after the avail ring
    > and the used ring, whatever the number of descriptors (which must be a
    > power of 2).
    >
    > We add a feature bit so the guest can tell the host that it's writing
    > out the current value there, if it wants to use that.
    >
    > This is based on a patch by Rusty Russell, with the main difference
    > being that we dedicate a feature bit to guest to tell the host it is
    > writing the used index. This way we don't need to force host to publish
    > the last available index until we have a use for it.
    >
    > Another difference is that while the feature helps virtio-net,
    > there have been conflicting reports wrt virtio-blk.
    > The reason is unknown, it could be due to the fact that
    > virtio-blk does not bother to disable interrupts at all.
    > So for now, this patch only acks this feature for -net.
    >
    > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

    It looks good.

    Later, Juan.


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