linux-kernel - Re: [PATCH 4/5] x86/mm/kaiser: Remove superfluous SWITCH_TO

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Message-ID: <b96a07a3-9fd5-f4c1-a4d5-433c590d006e@linux.intel.com>
Date:   Mon, 27 Nov 2017 14:47:08 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Cc:     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 4/5] x86/mm/kaiser: Remove superfluous SWITCH_TO_KERNEL

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.