Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath

From: Ingo Molnar
Date: Tue Mar 10 2015 - 09:21:59 EST

* Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:

> > So there are now +2 instructions (5 instead of 3) in the
> > system_call path, but there are -2 instructions in the SYSRETQ
> > path,
> Unfortunately, no. [...]

So I assumed that it was an equivalent transformation, given that none
of the changelogs spelled out the increase in overhead ...

> [...] There is only this change in SYSRETQ path, which simply
> changes where we get RSP from:
> @@ -293,7 +289,7 @@ ret_from_sys_call:
> CFI_REGISTER rip,rcx
> movq EFLAGS(%rsp),%r11
> /*CFI_REGISTER rflags,r11*/
> - movq PER_CPU_VAR(old_rsp), %rsp
> + movq RSP(%rsp),%rsp
> /*
> * 64bit SYSRET restores rip from rcx,
> * rflags from r11 (but RF and VM bits are forced to 0),
> Most likely, no change in execution speed here.
> At best, it is one cycle faster somewhere in address generation unit
> because for PER_CPU_VAR() address evaluation, GS base is nonzero.
> Since this patch does add two extra MOVs,
> I did benchmark these patches. They add exactly one cycle
> to system call code path on my Sandy Bridge CPU.

Hm, but that's the wrong direction, we should try to make it faster,
and to clean it up - but making it slower without really good reasons
isn't good.

Is 'usersp' really that much of a complication?


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