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Date: Mon, 27 Apr 2015 11:14:15 -0700
Message-ID: <CA+55aFySbPjeCgetxeSQPSP-h+yKnrHPcTGuoorAHJbCj53-8A@mail.gmail.com>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>, Andy Lutomirski <luto@kernel.org>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        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>
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On Mon, Apr 27, 2015 at 9:40 AM, Borislav Petkov <bp@alien8.de> wrote:
>
> Either way, the NOPs-version is faster and I'm running the test with the
> F16h-specific NOPs to see how they perform.

Btw, please don't use the "more than three 66h overrides" version.
Sure, that's what the optimization manual suggests if you want
single-instruction decode for all sizes up to 15 bytes, but I think
we're better off with the two-nop case for sizes 12-15) (4-byte nop
followed by 8-11 byte nop).

Because the "more than three 66b prefixes" really performs abysmally
on some cores, iirc.

                       Linus
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