Re: Xen crash: map_domain_page() on an NMI path

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 27/12/2013 07:29, Kai Huang wrote:
>
>
>
> On Fri, Dec 20, 2013 at 4:43 PM, Jan Beulich <JBeulich@suse.com
> <mailto:JBeulich@suse.com>> wrote:
>
>     >>> On 19.12.13 at 17:19, Andrew Cooper <andrew.cooper3@citrix.com
>     <mailto:andrew.cooper3@citrix.com>> wrote:
>     > However, for hardware pieces like this which are set up once at the
>     > start of day, and have the hardware pointed at a chosen region,
>     would it
>     > be acceptable to allocate their frames low enough to be covered
>     by the
>     > direct map area (protected by BUG()s?) and set up their base virtual
>     > addresses knowing that there will always be a valid mapping from
>     any Xen
>     > pagetables?  This seems better than constantly playing around
>     with the
>     > mappings.
>
>     That would still require further special casing in map_domain_page().
>
>     In the case here, and with 32-bit no longer a concern, a virtual
>     mapping should rather be obtained at boot time once and for all
>     using vmap().
>
>
> A question about map_domain_page. If I understand correctly, currently
> map_domain_page will still do page table setup with virtual address in
> mapcache area. Why can't we just map all physical memory to XEN's
>  virtual address slot, and do mfn_to_virt to get the virtual address?
>
> -Kai

Xen hands most of the upper canonical half to 64bit PV guest kernels.

The first 5TiB of RAM is unconditionally available via mfn_to_virt, but
Xen supports up to 16TiB of RAM.

Therefore, a server with more than 5TiB of RAM, or with RAM hoplug
regions above the 5TiB boundary require domain mappings to be accessed.

~Andrew

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