linux-kernel - Re: [PATCHv4 09/14] x86/mm: Handle boot-time paging mode switching at early boot

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Date:   Thu, 17 Aug 2017 11:05:05 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCHv4 09/14] x86/mm: Handle boot-time paging mode switching
 at early boot


* Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:

>  	/* Prepare to add new identity pagetables on demand. */
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index daf8936d0628..077e8b45784c 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -273,8 +273,20 @@ return_from_SYSCALL_64:
>  	 * Change top bits to match most significant bit (47th or 56th bit
>  	 * depending on paging mode) in the address.
>  	 */
> +#ifdef CONFIG_X86_5LEVEL
> +	testl	$1, p4d_folded(%rip)
> +	jnz	1f
> +	shl	$(64 - 57), %rcx
> +	sar	$(64 - 57), %rcx
> +	jmp	2f
> +1:
> +	shl	$(64 - 48), %rcx
> +	sar	$(64 - 48), %rcx
> +2:
> +#else
>  	shl	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
>  	sar	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> +#endif

So I really, really hate this change, as it slows down everything, not just LA57 
hardware, and slows down a critical system call hot path. (I expect enterprise 
distros to enable CONFIG_X86_5LEVEL.)

Could this use alternatives patching instead?

Thanks,

	Ingo