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

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Date:   Mon, 27 Nov 2017 22:43:08 +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 11:22:41AM +0100, Peter Zijlstra wrote:

> Could be a simple as this.. but I've not tested.
> 
> 
> 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);

Seems to work:

root@...-ep:~# rdmsr --all 0xc0000082 | uniq
ffffffff81beb780
root@...-ep:~# grep ffffffff81beb780 /proc/kallsyms 
ffffffff81beb780 T entry_SYSCALL_64