On Thu, Feb 23, 2023 at 5:23 AM Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> Some background first; on x86 platforms there is a free running TSC
> counter which can be used to generate extremely accurate profiling time
> stamps. Currently this can be used by BPF programs via hooking into perf
> subsystem and reading the value there; however this reduces the accuracy
> due to latency + jitter involved with long execution chain, and also the
> timebase gets converted into relative from the start of the execution of
> the program, instead of getting an absolute system level value.
Are you talking about rdtsc or some other counter?
Does it need an arch specific setup?
> Now, I do have a pretty trivial patch (under internal review atm. at
> Intel) that adds an x86 platform specific bpf helper that can directly
> read this timestamp counter without relying to perf subsystem hooks.
>
> Do people have any feedback / insights on this list about addition of
> such platform specific BPF helper, basically thumbs up/down for adding
> such a thing? Currently I don't think there are any platform specific
> helpers in the kernel.
Right. That's one of the reasons we don't add new helpers anymore.
Please use kfunc instead. You can add it to:
arch/x86/net/bpf_jit_comp.c
like:
__bpf_kfunc u64 bpf_read_rdtsc(void)
{ asm ("...
or to arch specific kernel module.
Make sure to add selftests when you submit a patch.
