Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

[Denys Vlasenko <dvlasenk@redhat.com>]

On 03/25/2015 10:28 AM, Ingo Molnar wrote:
> 
> * Andy Lutomirski <luto@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.