Date: Thu, 11 Sep 1997 10:41:39 +0930 From: Greg Lehey <grog@lemis.com> To: Sean Eric Fagan <sef@Kithrup.COM> Cc: gallatin@CS.Duke.EDU, emulation@FreeBSD.ORG, jkh@time.cdrom.com, mike@smith.net.au, sos@sos.freebsd.dk Subject: Re: Net posting: SCO gets Linux emulation Message-ID: <19970911104139.00351@lemis.com> In-Reply-To: <199709110105.SAA09151@kithrup.com>; from Sean Eric Fagan on Wed, Sep 10, 1997 at 06:05:10PM -0700 References: <199709110105.SAA09151@kithrup.com>
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On Wed, Sep 10, 1997 at 06:05:10PM -0700, Sean Eric Fagan wrote:
>>> Well, its somewhat interesting because it runs entirely in userland
>>> and traps system calls via a SEGV handler.
>> Ugh. Is this what we have come to expect of SCO?
>
> That is both unfair and rude. (Bit of a warning here... I've known the
> author ever since I interviewed at SCO, and I happen to like him.)
Well, in fact, I left the question open. I was wondering. Sure, it
works (well, the concept works, and I assume the emulator does), but
it's not exactly the way to get high performance.
> I looked at the program; it's interesting, but not terribly exciting.
> However, I might have done the iBCS2 emulation the same way, if I could
> have -- however, trapping the system call vector in a user-mode program is
> hard. If 386BSD had used a different entry vector...
It's a question of performance. If I understand it correctly, you do
a SIGSEGV for every system call.
> Mike also probably would have done the program using LDT manipulation,
> except for the fact that the IDT is global.
Yes, I'm sure that there are constraints which made him choose this
method.
>>> And because of this, I imagine that its a good bit slower. Also,
>>> their '$LINUX_ROOT' path remapping is interesting if only for its
>>> flexibility, but their choice of what paths to remap is very
>>> haphazard compared with the {Free,Net}BSD approach.
>> Doesn't sound like a serious implementation effort to me.
>
> To a large degree, it isn't. He did it as a quick&dirty way to be able to
> run the Linux port of Acrobat Reader (there is/was no SCO version), and
> apparantly convinced SCO to allow him to release it under a Berkeley-style
> license.
>
> All told, it probably took Mike about a day to write this.
OK, basically you're agreeing with me. My thoughts were "this is a
cheap way to get functionality. It won't bring performance". I've
written plenty of that category in my time, and some of them have been
great successes.
Greg
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