linux-kernel - Re: [patch 38/60] x86/fixmap: Move cpu entry area into a separate PMD

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Date:   Wed, 6 Dec 2017 19:57:58 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirsky <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, daniel.gruss@...k.tugraz.at
Subject: Re: [patch 38/60] x86/fixmap: Move cpu entry area into a separate PMD

On Mon, Dec 04, 2017 at 03:07:44PM +0100, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@...nel.org>
> 
> This allows the cpu entry area PMDs to be shared between the kernel and
> user space page tables.
> 
> [ tglx: Fixed bottom of by one and added guards so other fixmaps can be
>   	added later ]
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  arch/x86/include/asm/fixmap.h |   14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -134,16 +134,22 @@ enum fixed_addresses {
>  #ifdef CONFIG_PARAVIRT
>  	FIX_PARAVIRT_BOOTMAP,
>  #endif
> -	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
> -	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
>  #ifdef	CONFIG_X86_INTEL_MID
>  	FIX_LNW_VRTC,
>  #endif
> -	/* Fixmap entries to remap the GDTs, one per processor. */
> +	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
> +	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
> +
> +	/*
> +	 * Fixmap entries to remap the IDT, and the per cpu entry areas.
> +	 * Aligend to a PMD boundary.

"Aligned"

> +	 */
> +	FIX_USR_SHARED_TOP = round_up(FIX_TEXT_POKE0 + 1, PTRS_PER_PMD),
>  	FIX_CPU_ENTRY_AREA_TOP,
>  	FIX_CPU_ENTRY_AREA_BOTTOM = FIX_CPU_ENTRY_AREA_TOP + (CPU_ENTRY_AREA_PAGES * NR_CPUS) - 1,
> +	FIX_USR_SHARED_BOTTOM  = round_up(FIX_CPU_ENTRY_AREA_BOTTOM + 2, PTRS_PER_PMD) - 1,

So those look like this here:

FIX_TEXT_POKE0:			0x285, va: 0xffffffffff57a000
FIX_USR_SHARED_TOP:		0x400, va: 0xffffffffff3ff000
FIX_CPU_ENTRY_AREA_TOP:		0x401, va: 0xffffffffff3fe000
FIX_CPU_ENTRY_AREA_BOTTOM:	0x458, va: 0xffffffffff3a7000
FIX_USR_SHARED_BOTTOM:		0x5ff, va: 0xffffffffff200000

and FIX_CPU_ENTRY_AREA_TOP is the one PTE before the last 4K. But we
could just as well use the last one too, no? I.e.,

	FIX_USR_SHARED_TOP = round_up(FIX_TEXT_POKE0 + 1, PTRS_PER_PMD),
	FIX_CPU_ENTRY_AREA_TOP = FIX_USR_SHARED_TOP,

?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--