Messages in this thread | | | From | Michał Mirosław <> | Date | Wed, 18 May 2011 13:10:50 +0200 | Subject | Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice |
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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.
Is it that you mean keeping a reference after all skbs pointing to the pages are released?
>> 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.
Best Regards, Michał Mirosław -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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