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Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen

On 2015/8/4 22:37, Stefano Stabellini wrote:
On Tue, 4 Aug 2015, Shannon Zhao wrote:
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

Good start!

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
   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.

OK so far

   All these tables will be copied or mapped to guest memory.

Are they copied or mapped? Also I think we need to recalculate the

The above changed tables are copied to DOM0 and the reused tables(DSDT, SPCR, RSDP) are mapped to DOM0.

Yes, we need to recalculate the checksum.

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.

I think we need to specify which Linux entry point is called, that I
think will be the proper non-EFI kernel entry point, which requires MMU
off (see Documentation/efi-stub.txt in linux).

Also it would be better to write the full bindings of the generated
minimal DT, see http://marc.info/?l=linux-kernel&m=142362266626403&w=2
and Documentation/arm/uefi.txt in linux.

Ok, will add the example of the minimal DT.

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.

It would be nice to go down into more details and write the parameters
of the hypercalls in the doc as they will become a newly supported ABI.

OK, will add these informations.

The evtchnop would need to be called something like
EVTCHNOP_get_notification_irq and would need to be ARM specific (on x86
things are different).

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.

I think this approach is prone to failures. A driver could program a
device for DMA involving regions not yet mapped. As a consequence the
DMA operation would fail because the SMMU would stop the transaction.

   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

I suggested to exploit the bus_notifier callbacks and issue an hypercall
there. In the case of the PCI bus, we are already handling notifications
in drivers/xen/pci.c:xen_pci_notifier.

Once you have a struct pci_dev pointer in your hand, you can get the
MMIO regions from pdev->resource[bar].

Does that work?

I'm not sure. I just have a glance at it, not look into it deeply. Will have a look tomorrow.

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?

I think that is OK for now, but it is good for you to bring up this
point here.  Dom0 will ask Xen to remap interrupts for any devices
assigned to DomU created after Dom0.


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