linux-kernel - Re: [patch V2 5/5] x86/kaiser: Add boottime disable switch

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Date:   Mon, 27 Nov 2017 10:48:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch V2 5/5] x86/kaiser: Add boottime disable switch

On Mon, Nov 27, 2017 at 12:14:08AM +0100, Thomas Gleixner wrote:
> KAISER comes with overhead. The most expensive part is the CR3 switching in
> the entry code.
> 
> Add a command line parameter which allows to disable KAISER at boot time.
> 
> Most code pathes simply check a variable, but the entry code uses a static
> branch. The other code pathes cannot use a static branch because they are
> used before jump label patching is possible. Not an issue as the code
> pathes are not so performance sensitive as the entry/exit code.
> 
> This makes KAISER depend on JUMP_LABEL and on a GCC which supports
> it, but that's a resonable requirement.
> 
> The PGD allocation is still 8k when CONFIG_KAISER is enabled. This can be
> addressed on top of this.

So in patch 15 Andy notes that we should probably also disable the
SYSCALL trampoline when we disable KAISER.

  https://lkml.kernel.org/r/20171124172411.19476-16-mingo@kernel.org