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

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Message-ID: <20171127225306.6f7iueus7fq2hg7h@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Nov 2017 23:53:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, 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 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.