[PATCH 0/2] x86/BPF: Add new BPF helper call bpf_rdtsc

[Tero Kristo]

Hello,

This patch series adds a new x86 arch specific BPF helper, bpf_rdtsc()
which can be used for reading the hardware time stamp counter (TSC.)
Currently the same counter is directly accessible from userspace
(using RDTSC instruction), and kernel space using various rdtsc_*()
APIs, however eBPF lacks the support.

The main usage for the TSC counter is for various profiling and timing
purposes, getting accurate cycle counter values. The counter can be
currently read from BPF programs by using the existing perf subsystem
services (bpf_perf_event_read()), however its usage is cumbersome at
best. Additionally, the perf subsystem provides relative value only
for the counter, but absolute values are desired by some use cases
like Wult [1]. The absolute value of TSC can be read with BPF programs
currently via some kprobe / bpf_core_read() magic (see [2], [3], [4] for
example), but this relies on accessing kernel internals and is not
stable API, and is pretty cumbersome. Thus, this patch proposes a new
arch x86 specific BPF helper to avoid the above issues.

-Tero

[1] https://github.com/intel/wult
[2] https://github.com/intel/wult/blob/c92237c95b898498faf41e6644983102d1fe5156/helpers/wult-tdt-helper/tdt-bpf.c#L102
[3] https://github.com/intel/wult/blob/c92237c95b898498faf41e6644983102d1fe5156/helpers/wult-tdt-helper/tdt-bpf.c#L133
[4] https://github.com/intel/wult/blob/c92237c95b898498faf41e6644983102d1fe5156/helpers/wult-tdt-helper/tdt-bpf.c#L488