Re: [RFC] design: design doc for 1:1 direct-map

[Fam Zheng <fam@xxxxxxxxxx>]

On 2020-12-08 10:12, Jan Beulich wrote:
> On 08.12.2020 10:07, Julien Grall wrote:
> > On 08/12/2020 05:21, Penny Zheng wrote:
> >> --- /dev/null
> >> +++ b/docs/designs/1_1_direct-map.md
> >> @@ -0,0 +1,87 @@
> >> +# Preface
> >> +
> >> +The document is an early draft for direct-map memory map
> >> +(`guest physical == physical`) of domUs. And right now, it constrains to 
> >> ARM
> > 
> > s/constrains/limited/
> > 
> > Aside the interface to the user, you should be able to re-use the same 
> > code on x86. Note that because the memory layout on x86 is fixed (always 
> > starting at 0), you would only be able to have only one direct-mapped 
> > domain.
> 
> Even one seems challenging, if it's truly meant to have all of the
> domain's memory direct-mapped: The use of space in the first Mb is
> different between host and guest.

Speaking about the case of x86, we can still direct-map the ram regions
to the single direct-mapped DomU because neither Xen nor dom0 require
those low mem.

We don't worry about (i.e. don't direct-map) non-ram regions (or any
range that is not reported as usable ram from DomU's PoV (dictated by
e820 table), so those can be MMIO or arbitrary mapping with EPT.

Fam