Messages in this thread | | | Date | Wed, 25 Mar 2015 16:03:49 +0100 | From | Denys Vlasenko <> | Subject | Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss |
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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.
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