Re: [RFC] x86/cpu_entry_area: move part of it back to fixmap

[Nadav Amit]

at 10:02 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Thu, Oct 4, 2018 at 9:31 AM Nadav Amit <namit@xxxxxxxxxx> wrote:
>> at 7:11 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> 
>>> On Oct 3, 2018, at 9:59 PM, Nadav Amit <namit@xxxxxxxxxx> 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 itâs enough of a win to justify the added complexity, we can try it.
>>> 
>>> You can see some pros and cons in the changelog:
>>> 
>>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Ftip%2Fbf904d2762ee6fc1e4acfcb0772bbfb4a27ad8a6&amp;data=02%7C01%7Cnamit%40vmware.com%7C9996b2dd6f1745dce10b08d62a1b3f3e%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636742693864878787&amp;sdata=NW0R%2Fv5OahZlTbbNgnFk20sF4Wt1W0MDjtv9g1k%2BWdg%3D&amp;reserved=0
>> 
>> Err.. Thatâs 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. :)

Thatâs 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 donât 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 youâd prefer. Letâs see if Iâm right.