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Subject: Re: [PATCH] x86/vdso: Use non-serializing instruction rdtsc
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To:     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>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
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From:   Dave Hansen <dave.hansen@intel.com>
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On 5/15/23 23:52, Rong Tao wrote:
> Replacing rdtscp or 'lfence;rdtsc' with the non-serializable instruction
> rdtsc can achieve a 40% performance improvement with only a small loss of
> precision.

I think the minimum that can be done in a changelog like this is to
figure out _why_ a RDTSCP was in use.  There are a ton of things that
can make the kernel go faster, but not all of them are a good idea.

I assume that the folks that wrote this had good reason for not using
plain RSTSC.  What were those reasons?