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Date:   Tue, 16 May 2023 10:57:13 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Dave Hansen <dave.hansen@intel.com>, Rong Tao <rtoax@foxmail.com>,
        tglx@linutronix.de
CC:     Rong Tao <rongtao@cestc.cn>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/vdso: Use non-serializing instruction rdtsc
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On May 16, 2023 7:12:34 AM PDT, Dave Hansen <dave=2Ehansen@intel=2Ecom> wro=
te:
>On 5/15/23 23:52, Rong Tao wrote:
>> Replacing rdtscp or 'lfence;rdtsc' with the non-serializable instructio=
n
>> rdtsc can achieve a 40% performance improvement with only a small loss =
of
>> precision=2E
>
>I think the minimum that can be done in a changelog like this is to
>figure out _why_ a RDTSCP was in use=2E  There are a ton of things that
>can make the kernel go faster, but not all of them are a good idea=2E
>
>I assume that the folks that wrote this had good reason for not using
>plain RSTSC=2E  What were those reasons?

I believe the motivation is that it is atomic with reading the CPU number=
=2E