Re: [PATCH 02/24] x86/mm/kaiser: Prepare the x86/entry assembly code for entry/exit CR3 switching

[Peter Zijlstra]

On Mon, Nov 27, 2017 at 11:49:01AM +0100, Ingo Molnar wrote:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index caf74a1bb3de..df0152bee8a8 100644

> @@ -198,6 +201,14 @@ 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
>  
>  	/* Construct struct pt_regs on stack */

I think we can do away with this one, on account of never getting here
because of entry_SYSCALL_64_trampoline.