Re: [Xen-devel] [PATCH 6/7] docs: Document start_info changes in Xen 4.2.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-02-25 at 16:27 +0000, Konrad Rzeszutek Wilk wrote:
> The  25833:bb85bbccb1c9. "x86/32-on-64: adjust Dom0 initial page table layout"
> fixes a bug in the reported value of pt_base versus where the page tables
> actually start. This documents this in the start of the world header note.
> 
> This clarifies the implied understanding that the page table space is
> pointed by pt_base. As in it is ".. implied that the range of page-tables
> is the range [pt_base, pt_base + nr_pt_frames), whereas that that range
> here indeed is [pt_base - 2, pt_base -2 + nr_pt_frames)" (Jan Beulich).

Jan -- does the updated comment below correctly reflect this bug?

> 
> Also make it crystal clear that pt_base == %cr3.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  xen/include/public/xen.h |   12 +++++++++++-
>  1 files changed, 11 insertions(+), 1 deletions(-)
> 
> diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
> index 5e9bbcb..099ad24 100644
> --- a/xen/include/public/xen.h
> +++ b/xen/include/public/xen.h
> @@ -693,7 +693,7 @@ typedef struct shared_info shared_info_t;
>   *      c. list of allocated page frames [mfn_list, nr_pages]
>   *         (unless relocated due to XEN_ELFNOTE_INIT_P2M)
>   *      d. start_info_t structure        [register ESI (x86)]
> - *      e. bootstrap page tables         [pt_base, CR3 (x86)]
> + *      e. bootstrap page tables         [pt_base and CR3 (x86)]
>   *      f. bootstrap stack               [register ESP (x86)]
>   *  4. Bootstrap elements are packed together, but each is 4kB-aligned.
>   *  5. The initial ram disk may be omitted.
> @@ -705,6 +705,16 @@ typedef struct shared_info shared_info_t;
>   *  8. There is guaranteed to be at least 512kB padding after the final
>   *     bootstrap element. If necessary, the bootstrap virtual region is
>   *     extended by an extra 4MB to ensure this.
> + *
> + * Note: Prior to 25833:bb85bbccb1c9. ("x86/32-on-64 adjust Dom0 initial page
> + * table layout") a bug caused the pt_base (3.e above) and cr3 to not point
> + * to the start of the guest page tables (it was offset by two pages).
> + * This only manifested itself on 32-on-64 dom0 kernels and not 32-on-64 domU
> + * or 64-bit kernels of any colour. The page tables for a 32-on-64 dom0 got
> + * allocated in the order: 'first L1','first L2', 'first L3', so the offset
> + * to the page table base is by two pages back. The initial domain if it is
> + * 32-bit and runs under a 64-bit hypervisor should _NOT_ use two of the
> + * pages preceding pt_base and mark them as reserved/unused.
>   */
>  
>  #define MAX_GUEST_CMDLINE 1024



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