On 08.12.2020 11:22, Fam Zheng wrote: > 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.
For one, the very first page is considered special in x86 Xen. No guest should gain access to MFN 0, unless you first audit all code and address all the issues you find. And then there's also Xen's low-memory trampoline living there. Plus besides the BDA (at real-mode address 0040:0000) I suppose the EBDA also shouldn't be exposed to a guest, nor anything else that the host finds reserved in E820. IOW it would be the host E820 to dictate some of the guest E820 in such a case. Jan
