Re: [Patch V2] expand x86 arch_shared_info to support linear p2m list

[David Vrabel <david.vrabel@citrix.com>]

On 01/12/14 10:15, Jan Beulich wrote:
>>>> On 01.12.14 at 10:29, <JGross@suse.com> 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, the root of the page table root and a p2m generation count.
>> 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.
>>
>> Right now XENFEAT_virtual_p2m will not be set. This will change when
>> the Xen tools support the virtual mapped p2m list.
> 
> This seems wrong: XENFEAT_* only reflect hypervisor capabilities.
> I.e. the availability of the new functionality may need to be
> advertised another way - xenstore perhaps?

Xenstore doesn't work for dom0.

Shouldn't this be something the guest kernel reports using a ELF note bit?

When building a domain (either in Xen for dom0 or in the tools), the
builder may provide a linear p2m iff supported by the guest kernel and
then (and only then) can it provide a guest with > 512 GiB.

David