Date: Mon, 22 Aug 2016 18:55:58 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Slawa Olhovchenkov <slw@zxy.spb.ru> Cc: Bruce Evans <brde@optusnet.com.au>, Ed Schouten <ed@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r304555 - head/sys/compat/cloudabi Message-ID: <20160822184056.M1897@besplex.bde.org> In-Reply-To: <20160821170611.GC8192@zxy.spb.ru> References: <201608210741.u7L7fBnN075023@repo.freebsd.org> <20160821105207.GS22212@zxy.spb.ru> <20160821210751.J2219@besplex.bde.org> <20160821120016.GZ8192@zxy.spb.ru> <20160821223255.K2478@besplex.bde.org> <20160821131447.GA8192@zxy.spb.ru> <20160821232721.G2639@besplex.bde.org> <20160821135826.GB8192@zxy.spb.ru> <20160822022832.Q3214@besplex.bde.org> <20160821170611.GC8192@zxy.spb.ru>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20160822184056.M1897>