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

[Bruce Evans]

On Sun, 21 Aug 2016, Slawa Olhovchenkov wrote:

> 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?

No.  PSL_AC is ignored in kernel mode.

>> 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?

Not quite that short.  i386 has the 1-byte cli instruction for conveniently
setting the interrupt enable flag, but setting PSL_AC seems to take at
least 3 instructions and 6-7 bytes (pushf; orb $N,$M(%[re][bs]p); popf).

>>>>>> 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?

Could be, or the same language with a different ABI.

Bruce