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Date: Mon, 09 Mar 2015 10:44:23 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
        Denys Vlasenko <vda.linux@googlemail.com>
CC: Denys Vlasenko <dvlasenk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, 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>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
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On 03/09/2015 09:44 AM, Linus Torvalds wrote:
> 
> And remember: those zero-cost out-of-order branches turn quite
> expensive if they *ever* mispredict. Even a 5% mispredict rate is
> likely to mean "it's better to have a data dependency chain".
> 
> So it could easily go either way. I'm not convinced the old code is bad at all.
> 

I'm inclined to side with Linus here.  I'm hesitant to change this based
on pure speculation.

To answer Andy's question: I do believe we need espfix for V86 mode as well.

	-hpa