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Date: Sun, 15 Dec 2002 23:01:32 +0100
From: Pavel Machek <pavel@ucw.cz>
To: "Albert D. Cahalan" <acahalan@cs.uml.edu>
Cc: linux-kernel@vger.kernel.org, hpa@zytor.com, terje.eggestad@scali.com
Subject: Re: Intel P6 vs P7 system call performance
Message-ID: <20021215220132.GB6347@elf.ucw.cz>
References: <200212150406.gBF469M482759@saturn.cs.uml.edu>
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Hi!

> > As far as I know, though, the SYSENTER patch didn't deal with several of
> > the corner cases introduced by the generally weird SYSENTER instruction
> > (such as the fact that V86 tasks can execute it despite the fact there
> > is in general no way to resume execution of the V86 task afterwards.)
> >
> > In practice this means that vsyscalls is pretty much the only sensible
> > way to do this.  Also note that INT 80h will need to be supported
> > indefinitely.
> >
> > Personally, I wonder if it's worth the trouble, when x86-64 takes care
> > of the issue anyway :)
> 
> There is another way:
> 
> Have apps enter kernel mode via Intel's purposely undefined
> instruction, plus a few bytes of padding and identification.
> Require that this not cross a page boundry. When it faults,
> write the SYSENTER, INT 0x80, or SYSCALL as needed. Leave
> the page marked clean so it doesn't need to hit swap; if it
> gets paged in again it gets patched again.

Thats *very* dirty hack. vsyscalls seem cleaner than that.
								Pavel
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