linux-kernel - Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch

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Message-ID: <3933B95E-9FFB-4A8C-8691-82AE8CD77514@fb.com>
Date:   Fri, 3 Sep 2021 16:58:08 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     bpf <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kjain@...ux.ibm.com" <kjain@...ux.ibm.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper
 bpf_get_branch_snapshot



> On Sep 3, 2021, at 1:28 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote:
>> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
>> +{
>> +#ifndef CONFIG_X86
>> +	return -ENOENT;
>> +#else
>> +	static const u32 br_entry_size = sizeof(struct perf_branch_entry);
>> +	u32 entry_cnt = size / br_entry_size;
>> +
>> +	if (unlikely(flags))
>> +		return -EINVAL;
>> +
>> +	if (!buf || (size % br_entry_size != 0))
>> +		return -EINVAL;
>> +
>> +	entry_cnt = static_call(perf_snapshot_branch_stack)(buf, entry_cnt);
>> +
>> +	if (!entry_cnt)
>> +		return -ENOENT;
>> +
>> +	return entry_cnt * br_entry_size;
>> +#endif
>> +}
> 
> Do we really need that CONFIG_X86 thing? Seems rather bad practise.

The ifndef will save a few cycles on architectures that do not support
branch stack. I personally don't have very strong preference on either
way. 

Thanks,
Song