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

[Juergen Gross <jgross@suse.com>]

On 11/14/2014 12:41 PM, Andrew Cooper wrote:
> 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() ?

Yes. Not very different compared to today's mapping via the 3 level
p2m tree. Just another entry format, 4 instead of 3 levels and starting
at an offset.


Juergen