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

[Bruce Evans]

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:

%%%
#include <sys/types.h>
#include <machine/cpufunc.h>
#include <machine/psl.h>

int
main(void)
{
 	char ch[5];

 	write_eflags(read_eflags() | PSL_AC);
 	*(int *)&ch[0] = 0;
 	*(int *)&ch[1] = 1;
 	/* NOTREACHED */
}
%%%

This works on amd64 too after s/eflags/rflags.

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.

>>>> 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.

Bruce