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Date: Mon, 27 Nov 2017 10:48:46 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, daniel.gruss@iaik.tugraz.at,
        hughd@google.com, keescook@google.com, linux-mm@kvack.org,
        michael.schwarz@iaik.tugraz.at, moritz.lipp@iaik.tugraz.at,
        richard.fellner@student.tugraz.at
Subject: Re: [patch V2 5/5] x86/kaiser: Add boottime disable switch
Message-ID: <20171127094846.gl6zo3rftiyucvny@hirez.programming.kicks-ass.net>
References: <20171126231403.657575796@linutronix.de>
 <20171126232414.645128754@linutronix.de>
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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