[v4,00/10] x86/hyperv/hv_vtl: Use a wakeup mailbox to boot secondary CPUs

Hi,

Here is a new version of this series. Many thanks to Rob, Rafael,
Krzysztof, and Michael for your feedback. Previous versions can be found in
1], [2], and [3].

The biggest changes in this version are in the DeviceTree bindings and
their relationship with ACPI as well as relocating the ACPI wakeup code to
a new common file that both DeviceTree- and ACPI-based system can use. See
the changelog for details.

If the DeviceTree bindings look good, then the patches should be ready for
review by the x86, ACPI, and Hyper-V maintainers.

Thanks in advance for your feedback!

...

This patchset adds functionality to use a wakeup mailbox to boot secondary
CPUs in Hyper-V VTL level 2 TDX guests with virtual firmware that describes
hardware using a DeviceTree graph. Although this is the target use case,
the use of the mailbox depends solely on it being enumerated in the
DeviceTree.

On x86 platforms, secondary CPUs are typically booted using INIT assert,
de-assert followed by Start-Up IPI messages. Virtual machines can also make
hypercalls to bring up secondary CPUs to a desired execution state. These
two mechanisms require support from the hypervisor. Confidential computing
VMs in a TDX environment cannot use this mechanism because the hypervisor
is considered an untrusted entity.

Linux already supports the ACPI Multiprocessor Wakeup Structure in which
the guest platform firmware boots the secondary CPUs and transfers control
to the kernel using a mailbox. This mechanism does not need involvement
of the VMM. It can be used in a Hyper-V VTL level 2 TDX guest.

Currently, this mechanism can only be used on x86 platforms with firmware
that supports ACPI. There are platforms that use DeviceTree (e.g., OpenHCL
[4]) instead of ACPI to describe the hardware.

Provided that the wakeup mailbox enumerated in a DeviceTree-based platform
firmware is implemented as described in the ACPI specification, the kernel
can used common code for both DeviceTree and ACPI systems. The DeviceTree
firmware does not need to use any ACPI table to publish the mailbox.

This patcheset is structured as follows:

   * Relocate portions of the code handling the ACPI Multiprocessor Wakeup
     Structure code to a common location. (patches 1, 2)
   * Define DeviceTree bindings to enumerate a mailbox as described in
     the ACPI specification. (patch 3)
   * Find and setup the wakeup mailbox if found in the DeviceTree graph.
     (patch 4)
   * Prepare Hyper-V VTL2 TDX guests to use the Wakeup Mailbox to boot
     secondary CPUs when available. (patches 5-10)

I have tested this patchset on a Hyper-V host with VTL2 OpenHCL, QEMU, and
physical hardware.

Thanks and BR,
Ricardo

Changes since v3:
  - Added Reviewed-by: tags from Michael Kelley. Thanks!
  - Relocated the common wakeup code from acpi/madt_wakeup.c to a new
    smpwakeup.c to be used in DeviceTree- and ACPI-based systems.
  - Dropped the x86 CPU bindings as they are not a good fit to document
    firmware features.
  - Dropped the code that parsed and validated of the `enable-method`
    property for cpu@N nodes in x86. Instead, unconditionally parse and use
    the wakeup mailbox when found.
  - Updated the wakeup mailbox schema to avoid redefing the structure and
    operation of the mailbox. Instead, refer to the ACPI specification.
    Also clarified that the enumeration of the mailbox is done separately.
  - Prefixed helper functions of wakeup code with acpi_.

Changes since v2:
  - Only move out of the acpi directory acpi_wakeup_cpu() and its
    accessory variables. Use helper functions to access the mailbox as
    needed. This also fixed the warnings about unused code with CONFIG_
    ACPI=n that Michael reported.
  - Major rework of the DeviceTree bindings and schema. Now there is a
    reserved-memory binding for the mailbox as well as a new x86 CPU
    bindings. Both have `compatible` properties.
  - Rework of the code parsing the DeviceTree bindings for the mailbox.
    Now configuring the mailbox depends solely on its enumeration in the
    DeviceTree and not on Hyper-V VTL2 TDX guest.
  - Do not make reserving the first 1MB of memory optional. It is not
    needed and may introduce bugs.
  - Prepare Hyper-V VTL2 guests to unconditionally use the mailbox in TDX
    environments. If the mailbox is not available, booting secondary CPUs
    will fail gracefully.

Changes since v1:
  - Fix the cover letter's summary phrase.
  - Fix the DT binding document to pass validation.
  - Change the DT binding document to be ACPI independent.
  - Move ACPI-only functions into the #ifdef CONFIG_ACPI.
  - Change dtb_parse_mp_wake() to return mailbox physical address.
  - Rework the hv_is_private_mmio_tdx().
  - Remove unrelated real mode change from the patch that marks mailbox
    page private.
  - Check hv_isolation_type_tdx() instead of wakeup_mailbox_addr in
    hv_vtl_init_platform() because wakeup_mailbox_addr is not parsed yet.
  - Add memory range support to reserve_real_mode.
  - Remove realmode_reserve callback and use the memory range.
  - Move setting the real_mode_header to hv_vtl_init_platform.
  - Update comments and commit messages.
  - Minor style changes.

[1]. https://lore.kernel.org/r/20240806221237.1634126-1-yunhong.jiang@linux.intel.com
[2]. https://lore.kernel.org/r/20240823232327.2408869-1-yunhong.jiang@linux.intel.com
[3]. https://lore.kernel.org/r/20250503191515.24041-1-ricardo.neri-calderon@linux.intel.com
[4]. https://openvmm.dev/guide/user_guide/openhcl.html
--
2.43.0

---
Ricardo Neri (6):
      x86/acpi: Add a helper functions to setup and access the wakeup mailbox
      x86/acpi: Move acpi_wakeup_cpu() and helpers to smpwakeup.c
      dt-bindings: reserved-memory: Wakeup Mailbox for Intel processors
      x86/dt: Parse the Wakeup Mailbox for Intel processors
      x86/smpwakeup: Add a helper get the address of the wakeup mailbox
      x86/hyperv/vtl: Use the wakeup mailbox to boot secondary CPUs

Yunhong Jiang (4):
      x86/hyperv/vtl: Set real_mode_header in hv_vtl_init_platform()
      x86/realmode: Make the location of the trampoline configurable
      x86/hyperv/vtl: Setup the 64-bit trampoline for TDX guests
      x86/hyperv/vtl: Mark the wakeup mailbox page as private

 .../reserved-memory/intel,wakeup-mailbox.yaml      | 48 ++++++++++++
 arch/x86/Kconfig                                   |  7 ++
 arch/x86/hyperv/hv_vtl.c                           | 35 ++++++++-
 arch/x86/include/asm/smp.h                         |  4 +
 arch/x86/include/asm/x86_init.h                    |  3 +
 arch/x86/kernel/Makefile                           |  1 +
 arch/x86/kernel/acpi/madt_wakeup.c                 | 76 ++-----------------
 arch/x86/kernel/devicetree.c                       | 47 ++++++++++++
 arch/x86/kernel/smpwakeup.c                        | 88 ++++++++++++++++++++++
 arch/x86/kernel/x86_init.c                         |  3 +
 arch/x86/realmode/init.c                           |  7 +-
 11 files changed, 240 insertions(+), 79 deletions(-)
---
base-commit: 8858e8099446963ee6a0fb9f00f361dda52f04d5
change-id: 20250602-rneri-wakeup-mailbox-328efe72803f

Best regards,