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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.08.17 at 16:23, <jgross@xxxxxxxx> wrote:
> On 14/08/17 15:18, Jan Beulich wrote:
>> IOW the note needs to be present for a restriction to
>> be enforced, which in turn means the hypervisor first needs to
>> honor the note.
> 
> I don't think so. How would you get the note into already existing
> kernels having the restriction?

Well, we'd additionally need a guest config setting or some such.

>> Otherwise running a 4-level hypervisor on 5-level
>> capable hardware (with wider than 46-bit physical addresses)
>> would break Linux as well.
> 
> Right. OTOH on such a host bare metal 4-level Linux wouldn't run either
> (or only using less memory).
> 
> With Andrew's comment regarding grant v1 restricting _all_ current pv
> domains using this version to the first 16TB the complete discussion
> might be moot. So do we need an ELF note specifying whether a pv domain
> supports grant v2 in order to position it above 16TB? Or should this
> semantics be included in a kernel specifying its max physical address
> supported above 16TB?

First of all we'd need to enforce the 16Tb boundary in the
hypervisor. Then we could have a note relaxing this; whether
this is the note you propose or a separate one is secondary.

Jan


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