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SubjectRe: Intel P6 vs P7 system call performance
Linus Torvalds wrote:
> It shouldn't matter.
>
> NT is only tested by "iret", and if somebody sets NT in user space they
> get exactly what they deserve.

Indeed. I realized after I sent the previous mail that I had missed the
flags save/restore in switch_to :-(

Still, does this mean that there is some micro optimization opportunity in
the lcall7/lcall27 handlers to remove the popfl? After all TF is now
handled by some magic in do_debug unless I miss (again) something,
NT has become irrelevant, and cld in SAVE_ALL takes care of DF.

In short something like the following (I just love patches which only
remove code):

===== entry.S 1.51 vs edited =====
--- 1.51/arch/i386/kernel/entry.S Mon Jan 6 04:54:58 2003
+++ edited/entry.S Fri Jan 10 18:57:42 2003
@@ -156,16 +156,6 @@
movl %edx,EIP(%ebp) # Now we move them to their "normal" places
movl %ecx,CS(%ebp) #

- #
- # Call gates don't clear TF and NT in eflags like
- # traps do, so we need to do it ourselves.
- # %eax already contains eflags (but it may have
- # DF set, clear that also)
- #
- andl $~(DF_MASK | TF_MASK | NT_MASK),%eax
- pushl %eax
- popfl
-
andl $-8192, %ebp # GET_THREAD_INFO
movl TI_EXEC_DOMAIN(%ebp), %edx # Get the execution domain
call *4(%edx) # Call the lcall7 handler for the domain

>>For example, set NT and then execute sysenter with garbage in %eax, the
>>kernel will try to return (-ENOSYS) with iret and kill the task. As long
>>as it only allows a task to kill itself, it's not a big deal. But NT is
>>not cleared across task switches unless I miss something, and that looks
>>very dangerous.
>
>
> It _is_ cleared by task-switching these days. Or rather, it's saved and
> restored, so the original NT setter will get it restored when resumed.

Yeah, sorry for the noise.

>
>
>>I'm no Ingo, unfortunately, but you'll need at least the following patch
>>(the second hunk is only a typo fix) to the iret exception recovery code,
>>which used push and pops to get the smallest possible code size.
>
>
> Good job.

That was too easy since I did originally suggest the push/pop sequence :-)

Gabriel.



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