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Date: Mon, 27 Nov 2017 18:31:46 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 02/24] x86/mm/kaiser: Prepare the x86/entry assembly code
 for entry/exit CR3 switching
Message-ID: <20171127173146.plbdbiuzkv5ctgw6@hirez.programming.kicks-ass.net>
References: <20171127104923.14378-1-mingo@kernel.org>
 <20171127104923.14378-3-mingo@kernel.org>
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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.