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From: Brian Gerst <brgerst@gmail.com>
Date: Thu, 11 Jan 2018 10:22:43 -0500
Message-ID: <CAMzpN2iGjR=1aPp17gyEk2i=FCZ_KDfV+JoLzkHY980UNC4TZg@mail.gmail.com>
Subject: Re: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, gnomes@lxorguk.ukuu.org.uk,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>, rga@amazon.de,
        Tom Lendacky <thomas.lendacky@amd.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, Jan 11, 2018 at 9:32 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Jan 11, 2018 at 02:28:32PM +0000, David Woodhouse wrote:
>> On Thu, 2018-01-11 at 08:20 -0600, Josh Poimboeuf wrote:
>> >
>> > This seems weird.  I liked v1 a lot better.  What's the problem with
>> > patching in the whole thing?
>> >
>> > Also, if you go back to v1, it should be an easy objtool fix, just add
>> > ANNOTATE_NOSPEC_ALTERNATIVE in front of it.
>>
>> The objection was that I was patching in a fairly long set of
>> instructions. I confess I don't actually know why that's a problem,
>
> You get a giant string of NOPs, a single jmp should be faster.

That could be handled in add_nops(), where if over a certain threshold
it changes to a JMP.

--
Brian Gerst