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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 01:47:33PM +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.
    >
    > One more question: is userspace (or whatever is sending those packets)
    > denied from modifying passed pages? I assume it is, but just want to
    > be sure.
    >
    > Best Regards,
    > Michał Mirosław

    Good point.

    It's not denied in the sense that it still can modify them if it's
    buggy (the pages might not be read-only).
    But well-behaved userspace won't modify them until the callback
    is invoked.

    That would be a problem if the underlying device is
    a bridge where we might try to e.g. filter these packets -
    data can get modified after the filter. We'd have to copy
    whatever the filter accesses and use the copy - it's rarely
    the data itself.

    That's not normally a problem for macvtap connected to a physical NIC,
    as that already bypasses any and all filtering.

    But that's another limitation we should note in the comment,
    and another reason to limit to specific devices.

    --
    MST
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