Date: Sun, 21 Aug 2016 20:06:11 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Bruce Evans <brde@optusnet.com.au> Cc: 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: <20160821170611.GC8192@zxy.spb.ru> In-Reply-To: <20160822022832.Q3214@besplex.bde.org> 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>
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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?
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