Re: [PATCH 02/39] xen: document Xen is using an unused bit for the pagetables

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On Mon, Mar 26, 2012 at 07:45:49PM +0200, Andrea Arcangeli wrote:
> Xen has taken over the last reserved bit available for the pagetables
> which is set through ioremap, this documents it and makes the code
> more readable.

About a year ago we redid the P2M code to ditch the major use case for this.
But there were two left over cases that I hadn't found a good solution
for that would allow us to completly eliminate the use of this bit:

1). When setting a PTE of a PFN which overlaps an E820 hole or any of the
    non-E820-RAM entries, we lookup in the P2M and find out that
    the PFN is a 1:1 and return a pte.pte | pfn << PAGE_SIZE.

    But we also stick the _PAGE_IOMAP on it so that when the call to
    xen_pte_val is done we don't end up doing the lookup in the P2M tree
    once more and just set the pte as is.

    So this is the dance between xen_pte_val and xen_make_pte.

2). When the userspace tries to mmap a guest memory for save/migrate
    or to setup something in the guest, it would use the xen_remap_domain_mfn_range
    to setup PTE's with the guest's PFN (gpfn). The _PAGE_IOMAP
    is used again to tell xen_pte_val to not bother looking it up in the
    P2M tree and use it as is.

So.. any thoughts on how to eliminate the usage of this?
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/pgtable_types.h |   11 +++++++++--
>  1 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 013286a..b74cac9 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -17,7 +17,7 @@
>  #define _PAGE_BIT_PAT		7	/* on 4KB pages */
>  #define _PAGE_BIT_GLOBAL	8	/* Global TLB entry PPro+ */
>  #define _PAGE_BIT_UNUSED1	9	/* available for programmer */
> -#define _PAGE_BIT_IOMAP		10	/* flag used to indicate IO mapping */
> +#define _PAGE_BIT_UNUSED2	10
>  #define _PAGE_BIT_HIDDEN	11	/* hidden by kmemcheck */
>  #define _PAGE_BIT_PAT_LARGE	12	/* On 2MB or 1GB pages */
>  #define _PAGE_BIT_SPECIAL	_PAGE_BIT_UNUSED1
> @@ -41,7 +41,7 @@
>  #define _PAGE_PSE	(_AT(pteval_t, 1) << _PAGE_BIT_PSE)
>  #define _PAGE_GLOBAL	(_AT(pteval_t, 1) << _PAGE_BIT_GLOBAL)
>  #define _PAGE_UNUSED1	(_AT(pteval_t, 1) << _PAGE_BIT_UNUSED1)
> -#define _PAGE_IOMAP	(_AT(pteval_t, 1) << _PAGE_BIT_IOMAP)
> +#define _PAGE_UNUSED2	(_AT(pteval_t, 1) << _PAGE_BIT_UNUSED2)
>  #define _PAGE_PAT	(_AT(pteval_t, 1) << _PAGE_BIT_PAT)
>  #define _PAGE_PAT_LARGE (_AT(pteval_t, 1) << _PAGE_BIT_PAT_LARGE)
>  #define _PAGE_SPECIAL	(_AT(pteval_t, 1) << _PAGE_BIT_SPECIAL)
> @@ -49,6 +49,13 @@
>  #define _PAGE_SPLITTING	(_AT(pteval_t, 1) << _PAGE_BIT_SPLITTING)
>  #define __HAVE_ARCH_PTE_SPECIAL
>  
> +/* flag used to indicate IO mapping */
> +#ifdef CONFIG_XEN
> +#define _PAGE_IOMAP	(_AT(pteval_t, 1) << _PAGE_BIT_UNUSED2)
> +#else
> +#define _PAGE_IOMAP	(_AT(pteval_t, 0))
> +#endif
> +
>  #ifdef CONFIG_KMEMCHECK
>  #define _PAGE_HIDDEN	(_AT(pteval_t, 1) << _PAGE_BIT_HIDDEN)
>  #else
> --
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