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    SubjectRe: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice
    On Wed, May 25, 2011 at 03:49:40PM -0700, Shirley Ma wrote:
    > On Fri, 2011-05-20 at 02:41 +0300, Michael S. Tsirkin wrote:
    > > So the requirements are
    > > - data must be released in a timely fashion (e.g. unlike virtio-net
    > > tun or bridge)
    > The current patch doesn't enable tun zero-copy. tun will copy data It's
    > not an issue now.
    > We can disallow macvtap attach to bridge when
    > zero-copy is enabled.

    Attach macvtap to a tun device though. Or e.g. veth device ...
    So there should be so generic way to disable zerocopy.
    It can either be a whitelist or a blacklist.

    > > - SG support
    > > - HIGHDMA support (on arches where this makes sense)
    > This can be checked by device flags.

    OK, but pls note that SG can get turned off dynamically.

    > > - no filtering based on data (data is mapped in guest)
    > > - on fast path no calls to skb_copy, skb_clone, pskb_copy,
    > > pskb_expand_head as these are slow
    > Any calls to skb_copy, skb_clone, pskb_copy, pskb_expand_head will do a
    > copy. The performance should be the same as none zero-copy case before.

    I'm guessing a copy is cheaper than get_user_pages+copy+put_page.
    But maybe not by much. Care checking that?

    > I have done/tested the patch V6, will send it out for review tomorrow.
    > I am looking at where there are some cases, skb remains the same for
    > filtering.

    To reliably filter on data I think we'll need to copy it first, otherwise
    guest can change it. Most filters only look at the header though.

    > > First 2 requirements are a must, all other requirements
    > > are just dependencies to make sure zero copy will be faster
    > > than non zero copy.
    > > Using a new feature bit is probably the simplest approach to
    > > this. macvtap on top of most physical NICs most likely works
    > > correctly so it seems a bit more work than it needs to be,
    > > but it's also the safest one I think ...
    > For "macvtap/vhost zero-copy" we can use SG & HIGHDMA to enable it, it
    > looks safe to me once patching skb_copy, skb_clone, pskb_copy,
    > pskb_expand_head.
    > To extend zero-copy in other usages, we can have a new feature bit
    > later.
    > Is that reasonable?
    > Thanks
    > Shirley

    Is the problem is extra work needed to extend feature bits?


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