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    SubjectRe: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice
    On Wed, May 18, 2011 at 01:40:29PM +0200, Michał Mirosław wrote:
    > W dniu 18 maja 2011 13:17 użytkownik Michael S. Tsirkin
    > <mst@redhat.com> napisał:
    > > On Wed, May 18, 2011 at 01:10:50PM +0200, Michał Mirosław wrote:
    > >> 2011/5/18 Michael S. Tsirkin <mst@redhat.com>:
    > >> > On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote:
    > >> >> On Tue, 2011-05-17 at 23:48 +0200, Michał Mirosław wrote:
    > >> >> > 2011/5/17 Shirley Ma <mashirle@us.ibm.com>:
    > >> >> > > Hello Michael,
    > >> >> > >
    > >> >> > > Looks like to use a new flag requires more time/work. I am thinking
    > >> >> > > whether we can just use HIGHDMA flag to enable zero-copy in macvtap
    > >> >> > to
    > >> >> > > avoid the new flag for now since mavctap uses real NICs as lower
    > >> >> > device?
    > >> >> >
    > >> >> > Is there any other restriction besides requiring driver to not recycle
    > >> >> > the skb? Are there any drivers that recycle TX skbs?
    > >> >
    > >> > Not just recycling skbs, keeping reference to any of the pages in the
    > >> > skb. Another requirement is to invoke the callback
    > >> > in a timely fashion.  For example virtio-net doesn't limit the time until
    > >> > that happens (skbs are only freed when some other packet is
    > >> > transmitted), so we need to avoid zcopy for such (nested-virt)
    > >> > scenarious, right?
    > >>
    > >> Hmm. But every hardware driver supporting SG will keep reference to
    > >> the pages until the packet is sent (or DMA'd to the device). This can
    > >> take a long time if hardware queue happens to stall for some reason.
    > >
    > > That's a fundamental property of zero copy transmit.
    > > You can't let the application/guest reuse the memory until
    > > no one looks at it anymore.
    > >
    > >> Is it that you mean keeping a reference after all skbs pointing to the
    > >> pages are released?
    > > No one should reference the pages after the callback is invoked, yes.
    >
    > >> >> Not more other restrictions, skb clone is OK. pskb_expand_head() looks
    > >> >> OK to me from code review.
    > >> > Hmm. pskb_expand_head calls skb_release_data while keeping
    > >> > references to pages. How is that ok? What do I miss?
    > >> It's making copy of the skb_shinfo earlier, so the pages refcount
    > >> stays the same.
    > > Exactly. But the callback is invoked so the guest thinks it's ok to
    > > change this memory. If it does a corrupted packet will be sent out.
    >
    > Hmm. I tool a quick look at skb_clone(), and it looks like this
    > sequence will break this scheme:
    >
    > skb2 = skb_clone(skb...);
    > kfree_skb(skb) or pskb_expand_head(skb); /* callback called */
    > [use skb2, pages still referenced]
    > kfree_skb(skb); /* callback called again */
    > This sequence is common in bridge, might be in other places.
    >
    > Maybe this ubuf thing should just track clones? This will make it work
    > on all devices then.
    >
    > Best Regards,
    > Michał Mirosław

    Well bridge has the problem that packet might get anywhere and it's
    really hard to track. Same for tun - it can get queued forever.
    veth, loopback are all a problem I think.

    IOW we really want to limit this to real physical NICs
    which mostly all DTRT. Whitelisting them with a new flag
    is likely the most concervative approach, no?

    --
    MST
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