at 10:02 AM, Andy Lutomirski <l...@kernel.org> wrote: > On Thu, Oct 4, 2018 at 9:31 AM Nadav Amit <na...@vmware.com> wrote: >> at 7:11 AM, Andy Lutomirski <l...@amacapital.net> wrote: >> >>> On Oct 3, 2018, at 9:59 PM, Nadav Amit <na...@vmware.com> 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: >>> >>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Ftip%2Fbf904d2762ee6fc1e4acfcb0772bbfb4a27ad8a6&data=02%7C01%7Cnamit%40vmware.com%7C9996b2dd6f1745dce10b08d62a1b3f3e%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636742693864878787&sdata=NW0R%2Fv5OahZlTbbNgnFk20sF4Wt1W0MDjtv9g1k%2BWdg%3D&reserved=0 >> >> 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.
