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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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?

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.

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