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    SubjectRe: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel
    On Tue, Sep 14, 2010 at 11:49:03AM -0700, Shirley Ma wrote:
    > On Tue, 2010-09-14 at 20:27 +0200, Michael S. Tsirkin wrote:
    > > As others said, the harder issues for TX are in determining that it's
    > > safe
    > > to unpin the memory, and how much memory is it safe to pin to beging
    > > with. For RX we have some more complexity.
    >
    > I think unpin the memory is in kfree_skb() whenever the last reference
    > is gone for TX. What we discussed about here is when/how vhost get
    > notified to update ring buffer descriptors. Do I misunderstand something
    > here?

    Right, that's a better way to put it.

    > > Well it's up to you of course, but this is not what I would try:
    > > if you look at the
    > > patchset vhost changes is not the largest part of it,
    > > so this sounds a bit like effort duplication.
    > >
    > > TX only is also much less interesting than full zero copy.
    >
    > It's not true. From the performance, TX only has gained big improvement.
    > We need to identify how much gain from TX zero copy, and how much gain
    > from RX zero copy.

    I was speaking from the code point of view:
    since we'll want both TX and RX eventually it's nice to
    see that some thought was given to RX even if we only merge
    TX as a first step.

    From the product POV, RX is already slower (more interrupts, etc)
    than TX so speeding it up might be more important,
    but I agree, every bit helps.

    > > I think that you should be able to simply combine
    > > the two drivers together, add an ioctl to
    > > enable/disable zero copy mode of operation.
    >
    > That could work. But what's the purpose to have two drivers if one
    > driver can handle it?
    >
    > Thanks
    > Shirley

    This was just an idea: I thought it's a good way for people interested
    in this zero copy thing to combine forces and avoid making
    the same mistakes, but it's not a must of course.

    --
    MST


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