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Date: Sat, 19 Oct 2002 06:02:38 +0200
From: Andi Kleen <ak@muc.de>
To: Jeff Dike <jdike@karaya.com>
Cc: Andi Kleen <ak@muc.de>, john stultz <johnstul@us.ibm.com>,
       Linus Torvalds <torvalds@transmeta.com>, andrea <andrea@suse.de>,
       lkml <linux-kernel@vger.kernel.org>,
       george anzinger <george@mvista.com>,
       Stephen Hemminger <shemminger@osdl.org>, discuss@x86-64.org, aj@suse.de
Subject: Re: [PATCH] linux-2.5.43_vsyscall_A0
Message-ID: <20021019040238.GA21914@averell>
References: <20021019031002.GA16404@averell> <200210190450.XAA06161@ccure.karaya.com>
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[full quote for context]

On Sat, Oct 19, 2002 at 06:49:59AM +0200, Jeff Dike wrote:
> ak@muc.de said:
> > Guess you'll have some problems then with UML on x86-64, which always
> > uses vgettimeofday. But it's only used for gettimeofday() currently,
> > perhaps it's  not that bad when the UML child runs with the host's
> > time.
> 
> It's not horrible, but it's still broken.  There are people who depend
> on UML being able to keep its own time separately from the host.
> 
> > I guess it would be possible to add some support for UML to map own
> > code over the vsyscall reserved locations. UML would need to use the
> > syscalls then. But it'll be likely ugly. 
> 
> Yeah, it would be.
> 
> My preferred solution would be for libc to ask the kernel where the vsyscall
> area is.  That's reasonably clean and virtualizable.  Andrea doesn't like it
> because it adds a few instructions to the vsyscall address calculation.

I would have no problems with adding that to the x86-64 kernel. It could
be passed in by the ELF environment vector and added to the ABI. 
Overhead should be negligible, it just needs a single table lookup.  
Andreas, what do you think ? 

-Andi
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