Re: [PATCH V3 1/1] expand x86 arch_shared_info to support >3 level p2m tree

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

On 09/09/14 10:58, 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 64 bit pv domain.
> A 32 bit pv domain can support more, as each memory page can hold 1024
> instead of 512 entries, leading to a limit of 4 TB. To be able to
> support more RAM on x86-64 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).

A small domain feeling a little tight on space could easily opt for a 2
or even 1 level p2m.  (After all, one advantage of virt is to cram many
small VMs into a server).

How is xen and toolstack support for n-level p2ms going to be advertised
to guests?  Simply assuming the toolstack is capable of dealing with
this new scheme wont work with a new pv guest running on an older Xen.

~Andrew

>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  xen/include/public/arch-x86/xen.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/xen/include/public/arch-x86/xen.h b/xen/include/public/arch-x86/xen.h
> index f35804b..2ca996c 100644
> --- a/xen/include/public/arch-x86/xen.h
> +++ b/xen/include/public/arch-x86/xen.h
> @@ -224,7 +224,12 @@ 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 */
>  };
>  typedef struct arch_shared_info arch_shared_info_t;
>