Re: Intel P6 vs P7 system call performance

From: Albert D. Cahalan (
Date: Sat Dec 14 2002 - 23:06:09 EST

H. Peter Anvin writes:

> 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.

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