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

[Peter Zijlstra]

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