On 03/25/2015 10:28 AM, Ingo Molnar wrote:
>
> * Andy Lutomirski <l...@amacapital.net> wrote:
>
>> Now we can do a fun hack on top. On Intel, we have
>> sysenter/sysexitl and, on AMD, we have syscall/sysretl. But, if I
>> read the docs right, Intel has sysretl, too. So we can ditch
>> sysexit entirely, since this mechanism no longer has any need to
>> keep the entry and exit conventions matching.
>
> So this only affects 32-bit vdsos, because on 64-bit both Intel and
> AMD have and use SYSCALL/SYSRET.
>
> So my question would be: what's the performance difference between
> INT80 and sysenter entries on 32-bit, on modern CPUs?
>
> If it's not too horrible (say below 100 cycles) then we could say that
> we start out the simplification and robustification by switching Intel
> over to INT80 + SYSRET on 32-bit, and once we know the 32-bit SYSRET
> and all the other simplifications work fine we implement the
> SYSENTER-hack on top of that?
int 0x80 is about 250 cycles slower than syscall/sysenter.
(I mean, the instruction per se, not the full round-trip).
This looks too horrible to ignore :(
> Is there any user-space code that relies on being able to execute an
> open coded SYSENTER, or are we shielded via the vDSO?
Userspace can't use open-coded sysenter. It will return to a different
address.
Userspace _can_ do this:
my_sysenter:
push %ecx
push %edx
push %ebp
movl %esp,%ebp
sysenter
/* end of my_sysenter() */
...
...
...
call my_sysenter
but this depends on matching stack layout with one used by vDSO.
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