[lkml]   [2018]   [Oct]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC] x86/cpu_entry_area: move part of it back to fixmap
at 10:02 AM, Andy Lutomirski <> wrote:

> On Thu, Oct 4, 2018 at 9:31 AM Nadav Amit <> wrote:
>> at 7:11 AM, Andy Lutomirski <> wrote:
>>> On Oct 3, 2018, at 9:59 PM, Nadav Amit <> wrote:
>>>> This RFC proposes to return part of the entry-area back to the fixmap to
>>>> improve system-call performance. Currently, since the entry-area is
>>>> mapped far (more than 2GB) away from the kernel text, an indirect branch
>>>> is needed to jump from the trampoline into the kernel. Due to Spectre
>>>> v2, vulnerable CPUs need to use a retpoline, which introduces an
>>>> overhead of >20 cycles.
>>> That retpoline is gone in -tip. Can you see how your code stacks up against -tip? If it’s enough of a win to justify the added complexity, we can try it.
>>> You can see some pros and cons in the changelog:
>> Err.. That’s what I get for not following lkml. Very nice discussion.
>> Based on it, I may be able to do an additional micro-optimizations or
>> two. Let me give it a try.
> I think you should at least try to benchmark your code against mine,
> since you more or less implemented the alternative I suggested. :)

That’s what I meant. So I made a couple of tweaksin my implementation to
make as performant as possible. Eventually, there is a 2ns benefit for the
trampoline over the unified entry-path on average on my Haswell VM (254ns vs
256ns), yet there is some variance (1.2 & 1.5ns stdev correspondingly).

I don’t know whether such a difference should make one option to be preferred
over the other. I think it boils down to whether:

1. KASLR is needed.

2. Can you specialize the code-paths of trampoline/non-trampoline to gain
better performance. For example, by removing the ALTERNATIVE from
SWITCH_TO_KERNEL_CR3 and not reload CR3 on the non-trampoline path, you can
avoid an unconditional jmp on machines which are not vulnerable to Meltdown.

So I can guess what you’d prefer. Let’s see if I’m right.

 \ /
  Last update: 2018-10-05 20:35    [W:0.081 / U:24.436 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site