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Subject: Re: [PATCH 4/5] x86/mm/kaiser: Remove superfluous SWITCH_TO_KERNEL
To: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org
References: <20171127223110.479550152@infradead.org>
 <20171127223405.329572992@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, daniel.gruss@iaik.tugraz.at,
        hughd@google.com, keescook@google.com, linux-mm@kvack.org,
        michael.schwarz@iaik.tugraz.at, moritz.lipp@iaik.tugraz.at,
        richard.fellner@student.tugraz.at
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <b96a07a3-9fd5-f4c1-a4d5-433c590d006e@linux.intel.com>
Date: Mon, 27 Nov 2017 14:47:08 -0800
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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.