Message-ID: <5542B96E.2020400@zytor.com>
Date: Thu, 30 Apr 2015 16:23:26 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Andy Lutomirski <luto@amacapital.net>, Andy Lutomirski <luto@kernel.org>,
        X86 ML <x86@kernel.org>, Denys Vlasenko <vda.linux@googlemail.com>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@suse.com>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute
 issue
References: <20150427183854.GG28871@pd.tnic> <CA+55aFzYz=b887TLTuJ1u6Fv3B0eeXtnx+XzojJ8BBqm+Eha_g@mail.gmail.com> <20150427185344.GI28871@pd.tnic> <61BCF405-8000-43EB-A6B1-2BF5677E4ADE@zytor.com> <20150427200329.GL28871@pd.tnic> <2F6CA156-F03F-4F49-A6B9-7D1D1E1D805B@zytor.com> <20150428155511.GF19025@pd.tnic> <CA+55aFykHb8oyZquBq53MowOLzc5XXtNW4tad+4cTbO0YYFYNQ@mail.gmail.com> <20150428165807.GI19025@pd.tnic> <CA+55aFyKwsYy1jL31xEm1Do4t_ggVy5wwqMvcQ+Vzxko20UmRg@mail.gmail.com> <20150428183820.GJ19025@pd.tnic> <5542A0FB.50707@zytor.com>
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On 04/30/2015 02:39 PM, H. Peter Anvin wrote:
> This is the microbenchmark I used.
> 
> For the record, Intel's intention going forward is that 0F 1F will
> always be as fast or faster than any other alternative.
> 

I probably should have added that the microbenchmark specifically tests
for an atomic 5-byte NOP (as required by tracepoints etc.)  If the
requirement for 5-byte atomic is dropped there might be faster
combinations, e.g. 66 66 66 90 might work better on some platforms, or
using very long 0F 1F sequences.

	-hpa


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