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

[Juergen Gross <jgross@xxxxxxxx>]

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.

>> --- 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.
>> + */
>> +#define XEN_ELFNOTE_MAXPHYS_SIZE 19
> 
> 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.


Juergen


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