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SubjectRe: [RFC] x86/cpu_entry_area: move part of it back to fixmap
at 3:10 PM, Andy Lutomirski <> wrote:

> On Fri, Oct 5, 2018 at 3:08 PM Nadav Amit <> wrote:
>> 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.
> Why? KASLR is basically worthless on any existing CPU against
> attackers who can run local code.
>> 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.
> 2 ns isn't bad, at least on a non-PTI system. Which, I suppose, means
> that you should benchmark on AMD :)
> If the code is reasonably clean, I could get on board.

Fair enough. I’ll clean it and resend.


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