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Date:   Mon, 27 Nov 2017 12:50:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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, 27 Nov 2017, Peter Zijlstra wrote:
> On Mon, Nov 27, 2017 at 10:48:46AM +0100, Peter Zijlstra wrote:
> > 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
> 
> Could be a simple as this.. but I've not tested.

That's only one part of it. I think we need to fiddle with the exit side as
well.

Thanks,

	tglx

> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index f4f4ab8525bd..1be393a97421 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1442,7 +1442,10 @@ void syscall_init(void)
>  		(entry_SYSCALL_64_trampoline - _entry_trampoline);
>  
>  	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
> -	wrmsrl(MSR_LSTAR, SYSCALL64_entry_trampoline);
> +	if (kaiser_enabled)
> +		wrmsrl(MSR_LSTAR, SYSCALL64_entry_trampoline);
> +	else
> +		wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
>  
>  #ifdef CONFIG_IA32_EMULATION
>  	wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
> 

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