Design doc of adding ACPI support for arm64 on Xen

[Shannon Zhao <zhaoshenglong@huawei.com>]

This document is going to explain the design details of Xen booting with
ACPI on ARM. Maybe parts of it may not be appropriate. Any comments are
welcome.

To Xen itself booting with ACPI, this is similar to Linux kernel except
that Xen doesn't parse DSDT table. So I'll skip this part and focus on
how Xen prepares ACPI tables for DOM0 and how Xen passes them to DOM0.

1)copy and change some EFI and ACPI tables.
  a) Copy EFI_SYSTEM_TABLE and change the value of FirmwareVendor,
     VendorGuid, VendorTable, ConfigurationTable. These changes are not
     very special and it just assign values to these members.
  b) Create EFI_MEMORY_DESCRIPTOR table. This will add memory start and
     size information of DOM0. And DOM0 will get the memory information
     through this EFI table.
  c) Copy FADT table. Change the value of arm_boot_flags to enable PSCI
     and HVC. Let the hypervisor_id be "XenVMM" in order to tell DOM0
     that it runs on Xen hypervisor, so DOM0 can call hypercall to get
     some informations for booting necessity, such as grant tab start
     address and size. Change header revison, length and checksum as
     well.
  d) Copy GTDT table. Set non_secure_el2_interrupt and
     non_secure_el2_flags to 0 to mask EL2 timer for DOM0.
  e) Copy MADT table. According to the value of dom0_max_vcpus to change
     the number GICC entries.
  f) Create STAO table. This table is a new added one that's used to
     define a list of ACPI namespace names that are to be ignored by the
     OSPM in DOM0. Currently we use it to tell OSPM should ignore UART
     defined in SPCR table.
  g) Copy XSDT table. Add a new table entry for STAO and change other
     table's entries.
  h) Change the value of xsdt_physical_address in RSDP table.
  i) The reset of tables are not copied or changed. They are reused
     including DSDT, SPCR.
  All these tables will be copied or mapped to guest memory.

2)Create minimal DT to pass required informations to DOM0
  The minimal DT mainly passes DOM0 bootargs, address and size of initrd
  (if available), address and size of uefi system table, address and
  size of uefi memory table, uefi-mmap-desc-size and uefi-mmap-desc-ver.

3)DOM0 how to get grant table and event channel irq informations
  As said above, we assign the hypervisor_id be "XenVMM" to tell DOM0
  that it runs on Xen hypervisor. Then save the start address and size
  of grant table in domain->grant_table->start_addr and
  domain->grant_table->size. DOM0 can call a new hypercall
  GNTTABOP_get_start_addr to get these info.
  Same to event channel, we've already save interrupt number in
  d->arch.evtchn_irq, so DOM0 can call a new hypercall EVTCHNOP_get_irq
  to get the irq.

4)How to map MMIO regions
  a)Current implementation is mapping MMIO regions in Dom0 on demand
    when trapping in Xen with a data abort.
  b)Another way is to map all the non-ram memory regions before booting.
    But as suggested by Stefano, this will use a lot of memory to store
    the pagetables.
  c)Another suggested way is to use a hypercall from DOM0 to request
    MMIO regions mappings after Linux complete parsing the DSDT. But I
    didn't find a proper place to issue this call. Anyone has some
    suggestion?
5)How route device interrupt to DOM0
  Currently we route all the SPI interrupts to DOM0 before DOM0 booting.
  But this maybe a workaround. What's the right choice? After DOM0
  parses the interrupt information from DSDT and call a hypercall to
  route them?

Look forward to your comments.
Thanks,

-- 
Shannon