Date: Mon, 27 Apr 2015 20:47:30 +0200
From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
        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>, 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>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue
Message-ID: <20150427184730.GH28871@pd.tnic>
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 <20150425211206.GE32099@pd.tnic>
 <CALCETrWvqyCYOzCYXz7ZnzaM0obbidqo5CxVp2Hn1ELEiC3m3g@mail.gmail.com>
 <20150427085305.GB6774@pd.tnic>
 <20150427113506.GG6774@pd.tnic>
 <CA+55aFykNBrdHFSVeLftG7Ujx4Tk=MuUJXD2rNYn9kwL+kLN5w@mail.gmail.com>
 <553E5FD8.2070709@redhat.com>
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On Mon, Apr 27, 2015 at 11:12:05AM -0700, Linus Torvalds wrote:
> So if one or two cycles in this code doesn't matter, then why are we
> adding alternate instructions just to avoid a few ALU instructions and
> a conditional branch that predicts perfectly? And if it does matter,
> then the 6-byte option looks clearly better..

You know what? I haven't even measured the tree *without* Denys'
stricter RCX canonical-ness patch. All numbers so far are from 4.0+ with
tip/master ontop which has Denys' patch.

And I *should* measure once with plain 4.1-rc1 and then with Denys'
patch ontop to see whether this all jumping-thru-alternatives-hoops is
even worth it.

If nothing else, it was a nice exercise. :-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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