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I have been estimating the impact of the recently announced Intel bug on my packet processing application using netmap. So far, I have measured that I process about 50 packets per each poll() system call made, but this figure doesn't include gettimeofday() calls. I have also measured that I can read from a non-existing file descriptor (which is about the cheapest thing that a system call can do) 16.5 million times per second. My packet processing rate is 1.76 million packets per second, or in terms of system calls, 0.0352 million system calls per second. This means performance reduction would be 0.0352 / 16.5 = 0.21333% if system call penalty doubles, hardly something I should worry about.

However, my application may use gettimeofday() system calls quite often. My understanding is that these are not true system calls, but rather implemented as virtual system calls, as described in What are vdso and vsyscall?.

Now, my question is, does the fix to the recently announced Intel bug (that may affect ARM as well and that probably won't affect AMD) slow down gettimeofday() system calls? Or is gettimeofday() an entirely different animal due to being implemented as a different kind of virtual system call?

juhist
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2 Answers2

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In general, no.

The current patches keep things like the vDSO pages mapped in user-space, and only change the behavior for the remaining vast majority of kernel-only pages which will no longer be mapped in user-space.

On most architectures, gettimeofday() is implemented as a purely userspace call, and never enters the kernel, doesn't include the TLB flush or CR3 switch that KPTI implies, so you shouldn't see a performance impact.

Exceptions include unusual kernel or hardware configurations that don't use the vDSO mechanisms, e.g., if you don't have a constant rdtsc or if you have explicitly disabled rdtsc timekeeping via a boot parameter. You'd probably already know if that was the case since it means that gettimeofday would take 100-200ns rather than 15-20ns since it's already making a kernel call.

BeeOnRope
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Good question, the VDSO pages are kernel memory mapped into user space. If you single-step into gettimeofday(), you see a call into the VDSO page where some code there uses rdtsc and scales the result with scale factors it reads from another data page.

But these pages are supposed to be readable from user-space, so Linux can keep them mapped without any risk. The Meltdown vulnerability is that the U/S bit (user/supervisor) in page-table / TLB entries doesn't stop unprivileged loads (and further dependent instructions) from happening microarchitecturally, producing a change in the microarchitectural state which can then be read with cache-timing.

Peter Cordes
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