On Wed, Mar 11, 2015 at 4:35 AM, Andy Lutomirski <l...@amacapital.net> wrote:
>>> Comparison of object code:
>>> Old:
>>> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax
>>> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah
>>> 1ee: 8a 44 24 34 mov 0x34(%esp),%al
>>> 1f2: 25 03 04 02 00 and $0x20403,%eax
>>> 1f7: 3d 03 04 00 00 cmp $0x403,%eax
>>> 1fc: 74 0f je 20d <ldt_ss>
>>> New:
>>> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
>>> 1eb: 75 0e jne 1fb <restore_nocheck>
>>> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
>>> 1f2: 74 07 je 1fb <restore_nocheck>
>>> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
>>> 1f9: 75 0f jne 20a <ldt_ss>
>>
>> Please do some benchmarking of this: a tight loop of getpid or getppid
>> syscalls ought to be enough to be able to time this accurately.
>
> Before you benchmark, I think you should reorder it: check CS, then
> OLDSS, then EFLAGS. The case where CS & 3 == 0, OLDSS & 4 == 4, and
> EFLAGS & VM == VM should be *extremely* rare.
>
> I'm going to hold off on resending my sp0/sp1/ss cleanups until we
> resolve this -- if it turns out that your code is the same or faster
> and therefore gets merged, then I think that all the -8 crap can just
> be deleted instead of being fixed.
I thought that I got NAKed by Linus on this change?
Basically, he wants to retain that padding because it guards against
_future_ bugs_ where someone would touch pt_regs->ss
and it will fail, very rarely. I think it's a sound reason to retain
the padding.
Anyway, since you ask. I benchmarked current code against this patch:
restore_all_notrace:
#ifdef CONFIG_X86_ESPFIX32
- movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
- # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
- # are returning to the kernel.
- # See comments in process.c:copy_thread() for details.
- movb PT_OLDSS(%esp), %ah
- movb PT_CS(%esp), %al
- andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
- cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
CFI_REMEMBER_STATE
- je ldt_ss # returning to user-space with LDT SS
+ testb $3, PT_CS(%esp)
+ jz restore_nocheck # CPL0, not it
+#ifdef CONFIG_VM86
+ testb $2, (PT_EFLAGS+2)(%esp)
+ jnz restore_nocheck # EFLAGS.VM set, not it
+#endif
+ # Note: we access PT_OLDSS only when we know it exists.
+ # If PT_CS is from CPL0, it does not exist.
+ testb $SEGMENT_LDT, PT_OLDSS(%esp)
+ jnz ldt_ss # returning to user-space with LDT SS
#endif
This code only gets executed on int80 path, not on sysenter.
The test was to run 10 million getpids.
The numbers I've got are *the same* before and after the patch:
227.99 ns per getpid().
I double-checked my test setup by adding a few PAUSE insns
in this code - which, as expected, was seen easily in the test run.
I did not forget to enable ESPFIX32 and VM86 in the .config.
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