[CFR] mge driver / elf reloc

[John-Mark Gurney]

Warner Losh wrote this message on Mon, Jul 21, 2014 at 10:31 -0600:
> 
> On Jul 21, 2014, at 10:25 AM, John-Mark Gurney <jmg at funkthat.com> wrote:
> 
> > Warner Losh wrote this message on Mon, Jul 21, 2014 at 08:46 -0600:
> >> 
> >> On Jul 20, 2014, at 5:10 PM, John-Mark Gurney <jmg at funkthat.com> wrote:
> >> 
> >>> Tim Kientzle wrote this message on Sun, Jul 20, 2014 at 15:25 -0700:
> >>>> 
> >>>> On Jul 20, 2014, at 3:05 PM, John-Mark Gurney <jmg at funkthat.com> wrote:
> >>>> 
> >>>>> Ian Lepore wrote this message on Sat, Jul 19, 2014 at 16:54 -0600:
> >>>>>> Sorry to take so long to reply to this, I'm trying to get caught up.  I
> >>>>>> see you've already committed the mge fixes.  I think the ELF alignment
> >>>>>> fix looks good and should also be committed.
> >>>>> 
> >>>>> So, re the elf alignment...
> >>>>> 
> >>>>> I think we should get a set of macros that handle load/stores to/from
> >>>>> unaligned addresses that are transparent to the caller....  I need
> >>>>> these for some other code I'm writing... 
> >>>>> 
> >>>>> I thought Open/Net had these available, but I can't seem to find them
> >>>>> right now...
> >>>> 
> >>>> $ man 9 byteorder
> >>>> 
> >>>> is most of what you want, lacking only some aliases to pick
> >>>> the correct macro for native byte order.
> >>> 
> >>> Um, those doesn't help if you want native endian order?
> >> 
> >> Ummm, yes they do. enc converts from native order. dec decodes to native byte
> > 
> > No they don't.. If you want to read a value in memory that is native
> > endian order to native endian order (no conversion), they cannot be
> > used w/o using something like below?
> 
> Missed the native to native. bcopy works, but is ugly, as you note below.
> 
> >> order. They are more general cases than the ntoh* functions that are more traditional
> >> since they also work on byte streams that may not be completely aligned when
> >> sitting in memory. Which is what you are asking for.
> > 
> > So, you're saying that I now need to write code like:
> > #if LITTLE_ENDIAN /* or how ever this is spelled*/
> > 	var = le32enc(foo);
> > #else
> > 	var = be32enc(foo);
> > #endif
> > 
> > If I want to read a arch native endian value?  No thank you?
> 
> I?m not saying that at all.
> 
> >>> Also, only the enc/dec functions are documented to work on non-aligned
> >>> address, so that doesn't help in most cases?
> >> 
> >> They work on all addresses. They are even documented to work on any address:
> >> 
> >>     The be16enc(), be16dec(), be32enc(), be32dec(), be64enc(), be64dec(),
> >>     le16enc(), le16dec(), le32enc(), le32dec(), le64enc(), and le64dec()
> >>     functions encode and decode integers to/from byte strings on any align-
> >>     ment in big/little endian format.
> >> 
> >> So they are quite useful in general. Peeking under the covers at the implementation
> >> also shows they will work for any alignment, so I?m having trouble understanding
> >> where this objection is really coming from.
> > 
> > There are places where you write code such as:
> > 	int i;
> > 	memcpy(&i, inp, sizeof i);
> > 	/* use i */
> > 
> > In order to avoid alignment faults...  None of the functions in byteorder
> > do NO conversion of endian, or you have to know which endian you are but
> > that doesn't work on MI code...
> > 
> > Did you read what the commited code did?
> 
> No, I missed that bit beaded on your reply (which seemed to imply you needed
> endian conversion) which implied the enc/dec are only documented to work on non-aligned
> which is what I was correcting.

Hmm...  It appears that byteorder(9) leaks to userland though isn't
documented to be available in userland...  Should we fix this and make
it an offical userland API?  How to document it?  In my case, the
enc/dec version would have been enough for what I need, but not "part of
the userland API"...  There is an xref from byteorder(3), but no
comment on either that they will work..

> But maybe the more basic question is why do you even have packed
> structures that are native endian that you want to access as naturally
> aligned structures? How did they become native endian and why weren?t
> they converted to a more natural layout at that time?

The original message said why...

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

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