svn commit: r304555 - head/sys/compat/cloudabi

[Slawa Olhovchenkov]

On Mon, Aug 22, 2016 at 02:49:07AM +1000, Bruce Evans wrote:

> On Sun, 21 Aug 2016, Slawa Olhovchenkov wrote:
> 
> > On Sun, Aug 21, 2016 at 11:39:02PM +1000, Bruce Evans wrote:
> >
> >> On Sun, 21 Aug 2016, Slawa Olhovchenkov wrote:
> >>> I am remeber about platforms with missaligment trap when
> >>> accessing int16 by odd address. Now platforms like this do not exist
> >>> anymore?
> >>
> >> i386 still exists, and it supports trapping on misalignement for at least
> >> CPL 3 (not kernel CPL 0).  IIRC, amd64 drops support for this.
> >
> > Someone enable and support this? I am don't see.
> > May be PPC trap on this?
> > Alpha trap on this, but support of Alpha is droped.
> 
> It is a 1-line change in asm (or a little more in C with #includes) to
> enable the trap:

OK, we can turn amd64 in this mode.
And cat do request to kernel with unalligned access, this cause trap
and panic, yes?

> It is a trillion-line change to fix the compilers and applications to not
> do misaligned accesses :-).  I only tried to use this ~25 years ago.  Then
> the most obvious compiler bug was generating 32-bit acccesses to assign
> large but misaligned structs.  If the compiler just generated calls to
> memcpy(), that might work, but in practice libraries also assume alignment.

This issuse can be trigerred and by two-bytes assigmen, yes?

> >>>> There are also endianness problems.  The old version was even more broken
> >>>> on big endian systems.  The current version needs some magic to reverse
> >>>> the memcpy() of the bits.  We already depend on this for some 64-bit
> >>>> syscalls like lseek().
> >>>
> >>> Can you explain some more?
> >>> This is not transfer over network and don't read from external media.
> >>> Where is problem?
> >>
> >> It is similar to a network transfer.  It needs a protocol to pass values
> >> to applications.  Type puns are fragile even within a single compilation
> >> unit.
> >
> > Application ad kernel run with same byte order, not?
> 
> The application can do anything it wants, but has to translate if it uses
> the kernel or a library written in another language.

You talk about different byte order in differenr languages?