linux-kernel - Re: [PATCH v2] x86/mm/kaiser: Prepare the x86/entry assembly code for entry/exit CR3 switching

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Date:   Mon, 27 Nov 2017 07:29:48 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] x86/mm/kaiser: Prepare the x86/entry assembly code
 for entry/exit CR3 switching

On Sun, Nov 26, 2017 at 03:55:53PM +0100, Ingo Molnar wrote:
> @@ -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

I think it doesn't make sense to have a SWITCH_TO_KERNEL_CR3 here.  This
code is the (future) kaiser-disabled path, where SWITCH_TO_KERNEL_CR3 is
a no-op anyway.

-- 
Josh