linux-kernel - Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf

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Message-ID: <4b874e4c-4ad3-590d-3885-b4a3b894524e@linux.intel.com>
Date:   Thu, 6 Jul 2023 14:59:24 +0300
From:   Tero Kristo <tero.kristo@...ux.intel.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf_rdtsc


On 06/07/2023 08:16, John Fastabend wrote:
> Alexei Starovoitov wrote:
>> On Mon, Jul 3, 2023 at 3:58 AM Tero Kristo <tero.kristo@...ux.intel.com> 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@...ux.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)
>>> +{
>>> +       /* Check if Time Stamp is enabled only in ring 0 */
>>> +       if (cr4_read_shadow() & X86_CR4_TSD)
>>> +               return 0;
>> Why check this? It's always enabled in the kernel, no?

It is always enabled, but there are certain syscalls that can be used to 
disable the TSC access for oneself. prctl(PR_SET_TSC, ...) and 
seccomp(SET_MODE_STRICT,...). Not having the check in place would in 
theory allow a restricted BPF program to circumvent this (if there ever 
was such a thing.) But yes, I do agree this part is a bit debatable 
whether it should be there at all.


>>> +
>>> +       return rdtsc_ordered();
>> Why _ordered? Why not just rdtsc ?
>> Especially since you want to trace latency. Extra lfence will ruin
>> the measurements.
>>
> If we used it as a fast way to order events on multiple CPUs I
> guess we need the lfence? We use ktime_get_ns() now for things
> like this when we just need an order counter. We have also
> observed time going backwards with this and have heuristics
> to correct it but its rare.

Yeah, I think it is better to induce some extra latency instead of 
having some weird ordering issues with the timestamps.

Also, things like the ftrace also use rdtsc_ordered() as its underlying 
clock, if you use x86-tsc as the trace clock (see 
arch/x86/kernel/trace_clock.c.)

-Tero