On Fri, Oct 5, 2018 at 3:08 PM Nadav Amit <na...@vmware.com> wrote: > > 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.
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.
