Re: [PATCH] expand x86 arch_shared_info to support >3 level p2m tree

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/09/14 14:48, Juergen Gross wrote:
> The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list
> currently contains the mfn of the top level page frame of the 3 level
> p2m tree, which is used by the Xen tools during saving and restoring
> (and live migration) of pv domains. With three levels of the p2m tree
> it is possible to support up to 512 GB of RAM for a pv domain.

Specifically only 64bit PV domains have the 512GB limit.

32bit PV domains have a far larger supported RAM as they can fit twice
as many mfns in each p2m page.

>  To be
> able to support more RAM an additional level is to be added.
>
> This patch expands struct arch_shared_info with a new p2m tree root
> and the number of levels of the p2m tree. The new information is
> indicated by the domain to be valid by storing ~0UL into
> pfn_to_mfn_frame_list_list (this should be done only if more than
> three levels are needed, of course).
>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  xen/include/public/arch-x86/xen.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/xen/include/public/arch-x86/xen.h b/xen/include/public/arch-x86/xen.h
> index f35804b..b7fa2b6 100644
> --- a/xen/include/public/arch-x86/xen.h
> +++ b/xen/include/public/arch-x86/xen.h
> @@ -224,7 +224,13 @@ struct arch_shared_info {
>      /* Frame containing list of mfns containing list of mfns containing p2m. */
>      xen_pfn_t     pfn_to_mfn_frame_list_list;
>      unsigned long nmi_reason;
> -    uint64_t pad[32];
> +    /*
> +     * Following two fields are valid if pfn_to_mfn_frame_list_list contains
> +     * ~0UL.
> +     */
> +    unsigned long p2m_levels;   /* number of levels of p2m tree */
> +    xen_pfn_t     p2m_root;     /* p2m tree top level mfn */
> +    uint64_t pad[30];

This padding is now wrong, as unsigned long is only 4 bytes in 32bit,
and it is adjacent to another unsigned long.

~Andrew

>  };
>  typedef struct arch_shared_info arch_shared_info_t;
>