Re: [PATCH 4/5] x86/mm/kaiser: Remove superfluous SWITCH_TO_KERNEL

[Dave Hansen]

On 11/27/2017 02:31 PM, Peter Zijlstra wrote:
> We never use this code-path with KAISER enabled.
...
> @@ -201,14 +201,6 @@ ENTRY(entry_SYSCALL_64)
>  
>  	swapgs
>  	movq	%rsp, PER_CPU_VAR(rsp_scratch)
> -
> -	/*
> -	 * The kernel CR3 is needed to map the process stack, but we
> -	 * need a scratch register to be able to load CR3.  %rsp is
> -	 * clobberable right now, so use it as a scratch register.
> -	 * %rsp will look crazy here for a couple instructions.
> -	 */
> -	SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
>  	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp

What's the mechanism that we use to switch between the two versions of
the SYSCALL entry?  It wasn't obvious from some grepping.