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    SubjectRe: [PATCH 02/39] xen: document Xen is using an unused bit for the pagetables
    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@redhat.com>
    > ---
    > 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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