linux-kernel - Re: [PATCH] x86: use pgd accessors when cloning a pgd range.

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Date:	Wed, 27 Oct 2010 12:40:21 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ian Campbell <ian.campbell@...rix.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH] x86: use pgd accessors when cloning a pgd range.

On Wed, Oct 27, 2010 at 09:50:13AM +0100, Ian Campbell wrote:
> Page tables should always be updated using the proper accessor
> methods. Not doing so bypasses the paravirt infrastructure.
> 
> In this case the failure to do so was exposed under Xen by
> b40827fa7268 "x86-32, mm: Add an initial page table for core
> bootstrapping".
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: H. Peter Anvin <hpa@...ux.intel.com>
> Cc: Jeremy Fitzhardinge <jeremy@...p.org>
> ---
>  arch/x86/include/asm/pgtable.h |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index ada823a..0b4c514 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -619,7 +619,10 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm,
>   */
>  static inline void clone_pgd_range(pgd_t *dst, pgd_t *src, int count)
>  {
> -       memcpy(dst, src, count * sizeof(pgd_t));
> +	int i;
> +
> +	for (i=0; i<count; i++)
> +		set_pgd(&dst[i], src[i]);

Hmm, this slows down clone_pgd_range(). It is called at 3 sites total,
two of which happen only on boot in setup_arch() so they can be ignored
but the callchain

copy_process()
...
mm_init()
|->mm_alloc_pgd()
  |->pgd_alloc()
    |->pgd_ctor()
      |->clone_pgd_range()

could become noticeable. To be on the safe side, I'd make
clone_pgd_range() a macro calling either the native or the xen version..

hpa?

-- 
Regards/Gruss,
Boris.
--
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