Stephen Hemminger wrote:
You could play tricks with skb frags but it would be fragile
and not worth the trouble. The problem is that the receive
skb can stay in the system for a really long time (until the application
reads the data) so your fixed size buffer pool in hardware
would get exhausted.
Perhaps you could elaborate on why this is considered fragile? It seems to me
that as long as proper page management is performed, this mechanism should be
stable for processing non-contiguous receive buffers. I agree that
replenishment needs to be addressed, but I see that as an independent issue from
using frag lists on receive.
Also, if the h/w gives me a single packet in multiple
locations (i.e. non-contiguous chunks of memory), can
socket buffers handle chains of buffers? I am looking
for a facility like mbuf's in netbsd where one can
chain multiple buffers together to make construct a
single packet.
Yes, skb frag list could be used for that but you don't
want to do that. See above. Copy the data into an new
skb and reserve any necessary bytes so IP header is
aligned. I.e. if using ethernet header (14 bytes), do
skb_reserve(skb, 2) before copying the data.
But isn't avoiding the copy the point of all this? With some hardware models
using skbs as receive buffers is inconvenient and can be wasteful if jumbo
frames are required. Wouldn't a mechanism where it is possible to create (or
populate) an skb from data in a pre existing buffer without copying be more
general and address more hardware models? It seems to me that more and more
hardware is supporting scatter/gather output for received data, but the page*
model in the frag list can be restrictive in how data is placed before being
passed into the stack.
Thanks
Roy
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