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the attached patch implements SYSENTER/SYSEXIT based system calls for P6
type x86 CPUs that support it. (tested only on PII, - K6, PPro anyone?)
with this patch, null-syscall latency got more than halved on PIIs:
[mingo@hal fastcall]$ ./fastcall
(INT $0x80 based getpid(), got pid 497) latency:282 cycles
(SYSENTER based getpid(), got pid 497) latency:138 cycles
on a 266 MHz PII this is 0.51 usecs for a getpid(). (was 1.06 usecs)
the interface is binary compatible, glibc support with auto-detection is
in the works (being worked on by Richard Hendersson).
At first sight it looks like as if schedule() got slower by loading the
MSR in switch_to(), but actually scheduling times get faster, because no
matter how we get into schedule(), we have done it through some syscall
interface ...
the implementation has a few FIXME's, but is more or less equivalent to
the final version. (performance-wise)
i've attached some small testcode too which demonstrates the new syscall
interface, compile it with:
gcc -fomit-frame-pointer -o fastcall fastcall.c
enjoy, reports/comments/suggestions welcome.
-- mingo
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