linux-kernel - Re: [PATCH] xen: update mask_rw_pte after kernel page tables init changes

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Message-ID: <20110309174335.GH8049@dumpdata.com>
Date:	Wed, 9 Mar 2011 12:43:35 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	linux-kernel@...r.kernel.org, yinghai@...nel.org
Subject: Re: [PATCH] xen: update mask_rw_pte after kernel page tables init
 changes

On Wed, Mar 09, 2011 at 02:32:52PM +0000, Stefano Stabellini wrote:
> After "x86-64, mm: Put early page table high" already existing kernel
> page table pages can be mapped using early_ioremap too so we need to
> update mask_rw_pte to make sure these pages are still mapped RO.
> The reason why we have to do that is explain by the commit message of
> fef5ba797991f9335bcfc295942b684f9bf613a1:
> 
> "Xen requires that all pages containing pagetable entries to be mapped
> read-only.  If pages used for the initial pagetable are already mapped
> then we can change the mapping to RO.  However, if they are initially
> unmapped, we need to make sure that when they are later mapped, they
> are also mapped RO.
> 
> ..SNIP..
> 
> the pagetable setup code early_ioremaps the pages to write their
> entries, so we must make sure that mappings created in the early_ioremap
> fixmap area are mapped RW.  (Those mappings are removed before the pages
> are presented to Xen as pagetable pages.)"
> 
> We accomplish all this in mask_rw_pte by mapping RO all the pages mapped
> using early_ioremap apart from the last one that has been allocated
> because it is not a page table page yet (it has not been hooked into the
> page tables yet).
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

Also pls apply my SOB to "xen: set max_pfn_mapped to the last pfn mapped"

Thank you for tracking this one down.
> ---
>  arch/x86/xen/mmu.c |    8 +++++---
>  1 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 13783a1..5190af6 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -1440,10 +1440,12 @@ static __init pte_t mask_rw_pte(pte_t *ptep, pte_t pte)
>  	/*
>  	 * If the new pfn is within the range of the newly allocated
>  	 * kernel pagetable, and it isn't being mapped into an
> -	 * early_ioremap fixmap slot, make sure it is RO.
> +	 * early_ioremap fixmap slot as a freshly allocated page, make sure
> +	 * it is RO.
>  	 */
> -	if (!is_early_ioremap_ptep(ptep) &&
> -	    pfn >= pgt_buf_start && pfn < pgt_buf_end)
> +	if (((!is_early_ioremap_ptep(ptep) &&
> +			pfn >= pgt_buf_start && pfn < pgt_buf_end)) ||
> +			(is_early_ioremap_ptep(ptep) && pfn != (pgt_buf_end - 1)))
>  		pte = pte_wrprotect(pte);
>  
>  	return pte;
> -- 
> 1.5.6.5
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