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

[Juergen Gross <jgross@xxxxxxxx>]

On 14/08/17 14:36, Andrew Cooper wrote:
> On 14/08/17 11:21, Juergen Gross 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.
>>
>> As Xen needs to know whether a pv guest can handle such large addresses
>> the kernel of the guest has to advertise this capability.
>>
>> Add a new ELF note for the maximum physical address the kernel can
>> make use of.
>>
>> Please note that it is not required for a pv guest to support 5 level
>> paging in order to use high physical addresses.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> 
> Why?
> 
> With PAE paging, the maximum physical address width is 52 bits, and has
> been like that for a decade now.  5-level paging doesn't change this.
> 
> Are you saying that there is a Linux limitation where it doesn't cope
> properly with 52 bits of width in the pagetables?

Yes. See PTE_PFN_MASK in Linux kernel.

> A note like this is fine in principle if it is in fact needed, but I
> don't understand where the need arises.
> 
> ~Andrew
> 
> P.S. you are aware that all guests are constrained to 16TB anyway,
> because of the gnttab v1 32bit frame field?  In the case of PV guests,
> that’s the 16TB MFN boundary.

No, up to now I haven't thought of this.

This requires a major interface change, I guess. :-(


Juergen

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