Re: Intel P6 vs P7 system call performance

From: Linus Torvalds (torvalds@transmeta.com)
Date: Tue Dec 17 2002 - 13:24:44 EST


On Tue, 17 Dec 2002, Linus Torvalds wrote:
>
> Uli, how about I just add one ne warchitecture-specific ELF AT flag, which
> is the "base of sysinfo page". Right now that page is all zeroes except
> for the system call trampoline at the beginning, but we might want to add
> other system information to the page in the future (it is readable, after
> all).

Here's the suggested (totally untested as of yet) patch:

 - it moves the system call page to 0xffffe000 instead, leaving an
   unmapped page at the very top of the address space. So trying to
   dereference -1 will cause a SIGSEGV.

 - it adds the AT_SYSINFO elf entry on x86 that points to the system page.

Thus glibc startup should be able to just do

        ptr = default_int80_syscall;
        if (AT_SYSINFO entry found)
                ptr = value(AT_SYSINFO)

and then you can just do a

        call *ptr

to do a system call regardless of kernel version. This also allows the
kernel to later move the page around as it sees fit.

The advantage of using an AT_SYSINFO entry is that

 - no new system call needed to figure anything out
 - backwards compatibility (ie old kernels automatically detected)
 - I think glibc already parses the AT entries at startup anyway

so it _looks_ like a perfect way to do this.

                Linus

----
===== arch/i386/kernel/entry.S 1.42 vs edited =====
--- 1.42/arch/i386/kernel/entry.S	Mon Dec 16 21:39:04 2002
+++ edited/arch/i386/kernel/entry.S	Tue Dec 17 10:13:16 2002
@@ -232,7 +232,7 @@
 #endif

/* Points to after the "sysenter" instruction in the vsyscall page */ -#define SYSENTER_RETURN 0xfffff007 +#define SYSENTER_RETURN 0xffffe007

# sysenter call handler stub ALIGN ===== include/asm-i386/elf.h 1.3 vs edited ===== --- 1.3/include/asm-i386/elf.h Thu Oct 17 00:48:55 2002 +++ edited/include/asm-i386/elf.h Tue Dec 17 10:12:58 2002 @@ -100,6 +100,12 @@

#define ELF_PLATFORM (system_utsname.machine)

+/* + * Architecture-neutral AT_ values in 0-17, leave some room + * for more of them, start the x86-specific ones at 32. + */ +#define AT_SYSINFO 32 + #ifdef __KERNEL__ #define SET_PERSONALITY(ex, ibcs2) set_personality((ibcs2)?PER_SVR4:PER_LINUX)

@@ -115,6 +121,11 @@ extern void dump_smp_unlazy_fpu(void); #define ELF_CORE_SYNC dump_smp_unlazy_fpu #endif + +#define ARCH_DLINFO \ +do { \ + NEW_AUX_ENT(AT_SYSINFO, 0xffffe000); \ +} while (0)

#endif

===== include/asm-i386/fixmap.h 1.9 vs edited ===== --- 1.9/include/asm-i386/fixmap.h Mon Dec 16 21:39:04 2002 +++ edited/include/asm-i386/fixmap.h Tue Dec 17 10:11:31 2002 @@ -42,8 +42,8 @@ * task switches. */ enum fixed_addresses { - FIX_VSYSCALL, FIX_HOLE, + FIX_VSYSCALL, #ifdef CONFIG_X86_LOCAL_APIC FIX_APIC_BASE, /* local (CPU) APIC) -- required for SMP or not */ #endif

- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/



This archive was generated by hypermail 2b29 : Mon Dec 23 2002 - 22:00:17 EST