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

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

On 16/09/14 11:38, Juergen Gross wrote:
> On 09/16/2014 12:14 PM, David Vrabel wrote:
>> On 16/09/14 04:52, Juergen Gross wrote:
>>> On 09/15/2014 04:30 PM, David Vrabel wrote:
>>>> On 15/09/14 11:46, Juergen Gross wrote:
>>>>> So you'd prefer:
>>>>>
>>>>> 1) >512GB pv-domains (including Dom0) will be supported only with new
>>>>>      Xen (4.6?), no matter if the user requires migration to be
>>>>> supported
>>>>
>>>> Yes.  >512 GiB and not being able to migrate are not obviously related
>>>> from the point of view of the end user (unlike assigning a PCI device).
>>>>
>>>> Failing at domain save time is most likely too late for the end user.
>>>
>>> What would you think about following compromise:
>>>
>>> We add a flag that indicates support of multi-level p2m. Additionally
>>> the Linux kernel can ignore the flag not being set either if started as
>>> Dom0 or if told so via kernel parameter.
>>
>> This sounds fine but this override should be via the command line
>> parameter only.  Crash dump analysis tools may not understand the 4
>> level p2m.
>>
>>>>> to:
>>>>>
>>>>> 2) >512GB pv-domains (especially Dom0 and VMs with direct hw
>>>>> access) can
>>>>>      be started on current Xen versions, migration is possible only if
>>>>> Xen
>>>>>      is new (4.6?)
>>>>
>>>> There's also my preferred option:
>>>>
>>>> 3) >512 GiB PV domains are not supported.  Large guests must be PVH or
>>>> PVHVM.
>>>
>>> In theory okay, but not right now, I think. PVH Dom0 is not production
>>> ready.
>>
>> I'm not really seeing the need for such a large dom0.
> 
> Okay, then I'd come back to V1 of my patches. This is the minimum
> required to be able to boot up a system with Xen and more than 512GB
> memory without having to reduce the Dom0 memory via Xen boot parameter.
> 
> Otherwise the hypervisor built mfn_list mapped into the initial address
> space will be too large.
> 
> And no, I don't think setting the boot parameter is the solution here.
> Dom0 should be usable on a huge machine without special parameters.

Ok. The case where's dom0's p2m format matters is pretty specialized.

>> I also think a flat array for the p2m might be better (less complex).
>> There's plenty of virtual address space in a 64-bit guest to allow for
>> this.
> 
> Hmm, do you think we could reserve an area of many GBs for Xen in
> virtual space? I suspect this would be rejected as another "Xen-ism".

alloc_vm_area()

> BTW: the mfn_list_list will still be required to be built as a tree.

The tools could be given the guest virtual address and walk the guest
page tables.

This is probably too much of a difference from the existing ABI to be
worth pursuing at this point.

David