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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.12.14 at 14:11, <JGross@xxxxxxxx> wrote:
> On 12/01/2014 12:29 PM, Jan Beulich wrote:
>>>>> On 01.12.14 at 12:19, <david.vrabel@xxxxxxxxxx> wrote:
>>> On 01/12/14 10:15, Jan Beulich wrote:
>>>>>>> On 01.12.14 at 10:29, <JGross@xxxxxxxx> 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.
>>
>> Yes, surely this flag could act as a kernel capability indicator (via
>> the XEN_ELFNOTE_SUPPORTED_FEATURES note), like e.g.
>> XENFEAT_dom0 already does. JÃrgen's final statement, however,
>> suggested to me that this is meant to be only consumed by kernels.
> 
> Yes. The p2m list built by the domain builder is already linear. It may
> just be to small to hold all entries required e.g. for Dom0.
> 
> It's Xen-tools and kdump which have to deal with the linear p2m list.
> So the guest kernel has to be told if it is allowed to present the
> linear list instead of the 3-level tree at pfn_to_mfn_frame_list_list.
> 
> As this is true for Dom0 as well, this information must be given by the
> hypervisor.
> 
> I'm aware that XENFEAT_* is only used for hypervisor capabilities up to
> now. As the Xen tools are tightly coupled to the hypervisor I don't see
> why the features can't express the capability of the complete Xen
> installation instead. Would you prefer introducing another leaf for
> that purpose (submap.idx == 1) ?

That wouldn't change the odd situation of reporting a capability of
another component. That's even more of a problem for the Dom0
case, where the affected tool (kdump) isn't even under our control
(and shouldn't be).

But in the end - what's wrong with always (or conditionally upon a
CONFIG_* option and/or command line parameter and/or memory
size) filling both the old and new shared info fields? A capable tool
can determine whether the new one is valid, and an incapable tool
won't work on huge memory configs anyway.

Jan

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