Re: [PATCH 1/4] expand x86 arch_shared_info to support linear p2m list

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

On 14/11/14 09:37, 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 and for crash dump analysis. 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 switch to a virtual mapped
> p2m list.
>
> This patch expands struct arch_shared_info with a new p2m list virtual
> address and the mfn of the page table root. The new information is
> indicated by the domain to be valid by storing ~0UL into
> pfn_to_mfn_frame_list_list. The hypervisor indicates usability of this
> feature by a new flag XENFEAT_virtual_p2m.

How do you envisage this being used?  Are you expecting the tools to do
manual pagetable walks using xc_map_foreign_xxx() ?

~Andrew

> ---
>  xen/include/public/arch-x86/xen.h | 7 ++++++-
>  xen/include/public/features.h     | 3 +++
>  2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/xen/include/public/arch-x86/xen.h b/xen/include/public/arch-x86/xen.h
> index f35804b..b0f85a9 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_vaddr;    /* virtual address of the p2m list */
> +    unsigned long p2m_as_root;  /* mfn of the top level page table */
>  };
>  typedef struct arch_shared_info arch_shared_info_t;
>  
> diff --git a/xen/include/public/features.h b/xen/include/public/features.h
> index 16d92aa..ff0b82d 100644
> --- a/xen/include/public/features.h
> +++ b/xen/include/public/features.h
> @@ -99,6 +99,9 @@
>  #define XENFEAT_grant_map_identity        12
>   */
>  
> +/* x86: guest may specify virtual address of p2m list */
> +#define XENFEAT_virtual_p2m               13
> +
>  #define XENFEAT_NR_SUBMAPS 1
>  
>  #endif /* __XEN_PUBLIC_FEATURES_H__ */