linux-kernel - Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

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Message-ID: <7B08037D-1682-406D-90F1-C2B5B1899F7F@vmware.com>
Date:   Fri, 23 Mar 2018 19:12:50 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The entry/exit text and cpu_entry_area are mapped into userspace and
> the kernel.  But, they are not _PAGE_GLOBAL.  This creates unnecessary
> TLB misses.
> 
> Add the _PAGE_GLOBAL flag for these areas.
> 
> static void __init
> diff -puN arch/x86/mm/pti.c~kpti-why-no-global arch/x86/mm/pti.c
> --- a/arch/x86/mm/pti.c~kpti-why-no-global	2018-03-21 16:32:00.799192311 -0700
> +++ b/arch/x86/mm/pti.c	2018-03-21 16:32:00.803192311 -0700
> @@ -300,6 +300,13 @@ pti_clone_pmds(unsigned long start, unsi
> 			return;
> 
> 		/*
> +		 * Setting 'target_pmd' below creates a mapping in both
> +		 * the user and kernel page tables.  It is effectively
> +		 * global, so set it as global in both copies.
> +		 */
> +		*pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
if (boot_cpu_has(X86_FEATURE_PGE)) ?