svn commit: r257455 - head/sys/net

John-Mark Gurney jmg at funkthat.com
Thu Oct 31 22:14:59 UTC 2013


Luigi Rizzo wrote this message on Thu, Oct 31, 2013 at 22:13 +0100:
> On Thu, Oct 31, 2013 at 01:49:16PM -0700, John-Mark Gurney wrote:
> > Luigi Rizzo wrote this message on Thu, Oct 31, 2013 at 21:05 +0100:
> > > On Thu, Oct 31, 2013 at 01:27:25PM -0600, Ian Lepore wrote:
> > > ...
> > > > Is there any chance all this reworking might get us to a position where
> > > > the protocol header in an mbuf doesn't have to be 32-bit aligned
> > > > anymore?  We pay a pretty heavy price for that requirement in the
> > > > drivers of the least capable hardware we support, the systems that have
> > > > the least horsepower to spare to make an extra copy of each packet to
> > > > realign it.
> > > 
> > > So are you suggesting to use some 'copy_unaligned_32()' function/macro to
> > > access 32-bit protocol fields in the network stack ?
> > > (16-bit entries should not be an issue)
> > 
> > my idea has been to make a change to the various ip/tcp/udp layers
> > that is dependant upon __NO_STRICT_ALIGNMENT and if we do require
> > strict alignment to copy the header to a stack buffer to align the
> > data...
> 
> I'd rather use accessors functions/macros to read/write
> the unaligned headers so we can hide the #ifdefs in only
> one place.

I am/was trying to prevent massive code curn...

> The copy to a stack buffer is probably useful even for readability

Oh, I also realized I left out another part of it...

void
ip_input(struct mbuf *m)
{
#ifndef __NO_STRICT_ALIGNMENT
	struct ip tmpip;
#endif
	struct ip *ip = NULL;

#ifndef __NO_STRICT_ALIGNMENT
	bcopy(mtod(m, struct ip *), &tmpip, sizeof tmpip);
	ip = &tmpip;
#else
	ip = mtod(m, struct ip *);
#endif

}
> regardless of alignment (i do this in ipfw so i parse the
> packet only once, and those values are used often),
> but we should be careful to keep the copy in
> sync with the original in places where those headers are modified
> (NAT, ipfw fwd, maybe somewhere else ?)

If you modify the packet, you need to be careful to write back the
data when needed, i.e. passing the mbuf to another layer or
returning..  But that isn't anything too different than other
functions...

Looks like I'll have to revive my TS-7200 port for testing as npe on
AVILA apparently can handle the slightly odd alignment...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."


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