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

[Peter Zijlstra]

On Mon, Nov 27, 2017 at 02:47:08PM -0800, Dave Hansen wrote:
> 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.

the next patch, the code in tip will in fact never use this code.