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Re: [Xen-devel] [PATCH] include/public: add new elf note for support of huge physical addresses

On 14/08/17 12:48, Jan Beulich wrote:
>>>> On 14.08.17 at 12:35, <jgross@xxxxxxxx> wrote:
>> On 14/08/17 12:29, Jan Beulich wrote:
>>>>>> On 14.08.17 at 12:21, <jgross@xxxxxxxx> wrote:
>>>> Current pv guests will only see physical addresses up to 46 bits wide.
>>>> In order to be able to run on a host supporting 5 level paging and to
>>>> make use of any possible memory page there, physical addresses with up
>>>> to 52 bits have to be supported.
>>> Is this a Xen shortcoming or a Linux one (I assume the latter)?
>> It is a shortcoming of the Xen pv interface.
> Please be more precise: Where in the interface to we have a
> restriction to 46 bits?

We have no definition that the mfn width in a pte can be larger than
the pfn width for a given architecture (in this case a 4 level paging
64 bit x86 host).

So Xen has to assume a guest not telling otherwise has to be limited
to mfns not exceeding 4 level hosts maximum addresses.

Or would you like to not limit current pv guests to the lower 64TB and
risk them crashing, just because they interpreted the lack of any
specific mfn width definition in another way as you do?

>>>> --- a/xen/include/public/elfnote.h
>>>> +++ b/xen/include/public/elfnote.h
>>>> @@ -212,9 +212,18 @@
>>>>  #define XEN_ELFNOTE_PHYS32_ENTRY 18
>>>>  /*
>>>> + * Maximum physical address size the kernel can handle.
>>>> + *
>>>> + * All memory of the PV guest must be allocated below this boundary,
>>>> + * as the guest kernel can't handle page table entries with MFNs referring
>>>> + * to memory above this value.
>>>> + */
>>> Without use in the hypervisor or tools I don't see what good
>>> introducing this note will do.
>> The Linux kernel could make use of it from e.g. kernel 4.14 on. So in
>> case supports 5 level paging hosts lets say in Xen 4.12 it could run
>> Linux pv guests with kernel 4.14 making use of high memory addresses.
>> In case we don't define the note (or do it rather late) pv guests would
>> have to be restricted to the low 64TB of host memory.
> No matter what you say here - I can't see how defining the note
> alone will help.

It will help to introduce the support in Linux for large mfns _now_
instead of having to wait.

This can be easily compared to the support of 5 level paging in the
kernel happening right now: When the 5 level paging machines are
available in the future you won't be limited to a rather recent kernel,
but you can use one already being part of some distribution.


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