Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf_rdtsc

[Yonghong Song <yhs@meta.com>]

On 7/3/23 3:57 AM, Tero Kristo wrote:
> Currently the raw TSC counter can be read within kernel via rdtsc_ordered()
> and friends, and additionally even userspace has access to it via the
> RDTSC assembly instruction. BPF programs on the other hand don't have
> direct access to the TSC counter, but alternatively must go through the
> performance subsystem (bpf_perf_event_read), which only provides relative
> value compared to the start point of the program, and is also much slower
> than the direct read. Add a new BPF helper definition for bpf_rdtsc() which
> can be used for any accurate profiling needs.
> 
> A use-case for the new API is for example wakeup latency tracing via
> eBPF on Intel architecture, where it is extremely beneficial to be able
> to get raw TSC timestamps and compare these directly to the value
> programmed to the MSR_IA32_TSC_DEADLINE register. This way a direct
> latency value from the hardware interrupt to the execution of the
> interrupt handler can be calculated. Having the functionality within
> eBPF also has added benefits of allowing to filter any other relevant
> data like C-state residency values, and also to drop any irrelevant
> data points directly in the kernel context, without passing all the
> data to userspace for post-processing.
> 
> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
> ---
>   arch/x86/include/asm/msr.h |  1 +
>   arch/x86/kernel/tsc.c      | 23 +++++++++++++++++++++++
>   2 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 65ec1965cd28..3dde673cb563 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -309,6 +309,7 @@ struct msr *msrs_alloc(void);
>   void msrs_free(struct msr *msrs);
>   int msr_set_bit(u32 msr, u8 bit);
>   int msr_clear_bit(u32 msr, u8 bit);
> +u64 bpf_rdtsc(void);
>   
>   #ifdef CONFIG_SMP
>   int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 344698852146..ded857abef81 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -15,6 +15,8 @@
>   #include <linux/timex.h>
>   #include <linux/static_key.h>
>   #include <linux/static_call.h>
> +#include <linux/btf.h>
> +#include <linux/btf_ids.h>
>   
>   #include <asm/hpet.h>
>   #include <asm/timer.h>
> @@ -29,6 +31,7 @@
>   #include <asm/intel-family.h>
>   #include <asm/i8259.h>
>   #include <asm/uv/uv.h>
> +#include <asm/tlbflush.h>
>   
>   unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
>   EXPORT_SYMBOL(cpu_khz);
> @@ -1551,6 +1554,24 @@ void __init tsc_early_init(void)
>   	tsc_enable_sched_clock();
>   }
>   
> +u64 bpf_rdtsc(void)

Please see kernel/bpf/helpers.c. For kfunc definition, we should have

__diag_push();
__diag_ignore_all("-Wmissing-prototypes",
                   "Global functions as their definitions will be in 
vmlinux BTF");

_bpf_kfunc u64 bpf_rdtsc(void)
{
	...
}

__diag_pop();


> +{
> +	/* Check if Time Stamp is enabled only in ring 0 */
> +	if (cr4_read_shadow() & X86_CR4_TSD)
> +		return 0;
> +
> +	return rdtsc_ordered();
> +}
> +
> +BTF_SET8_START(tsc_bpf_kfunc_ids)
> +BTF_ID_FLAGS(func, bpf_rdtsc)
> +BTF_SET8_END(tsc_bpf_kfunc_ids)
> +
> +static const struct btf_kfunc_id_set tsc_bpf_kfunc_set = {
> +	.owner		= THIS_MODULE,
> +	.set		= &tsc_bpf_kfunc_ids,
> +};
> +
>   void __init tsc_init(void)
>   {
>   	if (!cpu_feature_enabled(X86_FEATURE_TSC)) {
> @@ -1594,6 +1615,8 @@ void __init tsc_init(void)
>   
>   	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
>   	detect_art();
> +
> +	register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tsc_bpf_kfunc_set);

register_btf_kfunc_id_set() could fail, maybe you at least wants to
have a warning so bpf prog users may be aware that kfunc bpf_rdtsc()
not really available to bpf programs?

>   }
>   
>   #ifdef CONFIG_SMP