[PATCH v5 00/15] x86: introduce NS16550-compatible UART emulator

[dmukhin@xxxxxxx]

x86 port of Xen lacks vUART facility similar to Arm's vpl011 to support x86
guest OS bring up in the embedded setups.

This patch series introduces initial in-hypervisor emulator for
NS8250/NS16x50-compatible UARTs under CONFIG_VUART_NS16X50.

In parallel domain creation scenario (hyperlaunch), NS16550 emulator helps
early guest firmware and OS bringup debugging, because it eliminates
dependency on the external emulator (qemu) being operational by the time
domains are created.

The emulator also allows to forward the physical console input to the x86
domain which is useful when a system has only one physical UART for early
debugging and this UART is owned by Xen.

By default, CONFIG_VUART_NS16X50 enables emulation of NS16550 at I/O port
0x2f8, IRQ#3 in guest OS (legacy COM2). Legacy COM resources cannot be
selected at built-time or via per-domain xl configuration in this initial
submission.

CONFIG_VUART_NS16X50_DEBUG enables some extra debugging facilities useful
for NS16550 emulator development/debugging (disabled by default).

The NS16550 emulator is disabled in default x86 configuration and goes under
CONFIG_EXPERT in Kconfig.

Limitations
===========
- Only x86;
- Only HVM domains support (build-time), PVH domains are not supported yet;
- Only legacy COM{1,2,3,4} resources via Kconfig, custom I/O ports/IRQs
  are not supported;
- Only Xen console as a backend, no inter-domain communication (similar to
  vpl011 on Arm);
- Only 8n1 emulation (8-bit data, no parity, 1 stop bit);
- No toolstack integration;
- No baud rate emulation (reports 115200 baud to the guest OS);
- No FIFO-less mode emulation;
- No RX FIFO interrupt moderation (FCR) emulation;
- No integration w/ VM snapshotting (HVM_REGISTER_SAVE_RESTORE() and
  friends);
- No ISA IRQ sharing allowed;
- No MMIO-based UART emulation.

Series
======

  Patch 1 introduces the new vUART framework, that is the code originally
  posted here:
    https://lore.kernel.org/xen-devel/20250624035443.344099-16-dmukhin@xxxxxxxx/
  Required for emulator.

  Patch 2 adds missing NS16550 definitions, required for emulator.

  Patch 3 introduces the basic emulator skeleton - state machine
  initialization stubs, I/O port handler stub, logging, etc.

  Patches 4-10 incrementally populate the minimal NS16550 register emulation.

  Patch 11 hooks vUART to console driver and enables physical console intput
  forwarding.

  Patch 12 hooks vUART state debugging (disabled by default).

  Pathes 13-15 introduce necessary changes to enable NS16550 on dom0 (and PVH).

Link to CI: 
https://gitlab.com/xen-project/people/dmukhin/xen/-/pipelines/2009685553
Link to branch: 
https://gitlab.com/xen-project/people/dmukhin/xen/-/tree/vuart-ns8250-v5?ref_type=heads

Testing
=======

  ```shell
  echo CONFIG_EXPERT=y >> .config
  echo CONFIG_VUART_NS16X50=y >> .config
  make olddefconfig
  ```
  COM2 (0x2f8) resources are used by default.

  To test w/ virtual COM2, the guest kernel parameters should contain
  something like the following:
    earlycon=uart,io,0x2f8,115200n8 console=uart,io,0x2f8,115200n8

  HVM
  ---
  Tested only boot of HVM linux guest with OVMF as the virtual firmware.
  SeaBIOS as a virtual firmware is not tested.

  PVH (dom0)
  ----------
  Xen is able to forward physical console input to the domain with virtual
  NS16550. To switch the console focus press Ctrl+aaa.
  Switch is limited on x86 to dom0 and Xen (fixes pending).
  With COM2, Linux kernel complains nobody cared about IRQ#3. Likely that's
  because guest DSDT needs patching, which is a follow on change, but not
  critical for demonstration; the code is disabled in default config.

Changes since v4:
- Split the series to make it simpler to review.
- Addressed feedback from v4.
- Dropped xl changes, which I will submit separately.
- Link to v4: 
https://lore.kernel.org/xen-devel/20250731192130.3948419-1-dmukhin@xxxxxxxx/

Changes since v3:
- Reduced the blast radius of the series, thanks to reviews, individual
  aspects (like console focus) touched in v3 moved to separate threads.
- Kept the UART emulator framework since I need to redo some of emulator code
  and there's more-or-less agreement on it (where to place, naming, scope).
- Applied the feedback from
    https://lore.kernel.org/xen-devel/20250624035443.344099-1-dmukhin@xxxxxxxx/
- Link to v3: 
https://lore.kernel.org/xen-devel/20250103-vuart-ns8250-v3-v1-0-c5d36b31d66c@xxxxxxxx/

Changes since v2:
- renamed emulator s/NS8250/NS16550/g
- reduced the patch series after addressing v2 feedback
- introduced driver framework for UART emulators
- unified guest OS printouts across all available UART emulators
- Link to v2: 
https://lore.kernel.org/xen-devel/20241205-vuart-ns8250-v1-0-e9aa923127eb@xxxxxxxx/

Changes since v1:
- dropped kmalloc/kfree aliases
- fixed ECLAIR jobs (thanks Andrew Cooper)
- addressed console forwarding on arm32 and arm64 (thanks to Luca Fancellu)
- moved NS8250 debugging stubs into its own patch
- added fix for https://gitlab.com/xen-project/xen/-/issues/184
- Link to v1: 
https://lore.kernel.org/r/20241126-vuart-ns8250-v1-v1-0-87b9a8375b7a@xxxxxxxx

Denis Mukhin (15):
  emul/vuart: introduce framework for UART emulators
  xen/8250-uart: update definitions
  emul/ns16x50: implement emulator stub
  emul/ns16x50: implement DLL/DLM registers
  emul/ns16x50: implement EIR/IIR registers
  emul/ns16x50: implement THR/RBR registers
  emul/ns16x50: implement FCR register (write-only)
  emul/ns16x50: implement LCR/LSR registers
  emul/ns16x50: implement MCR/MSR registers
  emul/ns16x50: implement SCR register
  emul/ns16x50: implement put_rx() hook
  emul/ns16550: implement dump_state() hook
  x86/domain: enable per-domain I/O port bitmaps
  xen/domain: allocate d->irq_caps before arch-specific initialization
  emul/ns16x50: implement IRQ emulation via vIOAPIC

 xen/arch/arm/xen.lds.S                   |   1 +
 xen/arch/ppc/xen.lds.S                   |   1 +
 xen/arch/riscv/xen.lds.S                 |   1 +
 xen/arch/x86/Makefile                    |   1 +
 xen/arch/x86/dom0_build.c                | 112 +--
 xen/arch/x86/domain.c                    |   2 +-
 xen/arch/x86/hvm/dom0_build.c            |   7 +
 xen/arch/x86/hvm/hvm.c                   |  55 +-
 xen/arch/x86/hvm/nestedhvm.c             |   8 +-
 xen/arch/x86/hvm/quirks.c                |   3 -
 xen/arch/x86/hvm/svm/nestedsvm.c         |   2 +-
 xen/arch/x86/hvm/vioapic.c               |  10 +
 xen/arch/x86/hvm/vmx/vvmx.c              |   4 +-
 xen/arch/x86/include/asm/hvm/nestedhvm.h |   3 +-
 xen/arch/x86/include/asm/hvm/support.h   |   2 -
 xen/arch/x86/include/asm/iocap.h         |   2 +
 xen/arch/x86/include/asm/irq.h           |   5 +-
 xen/arch/x86/ioport.c                    | 163 ++++
 xen/arch/x86/irq.c                       |  12 +-
 xen/arch/x86/physdev.c                   |   8 +-
 xen/arch/x86/pv/dom0_build.c             |   7 +
 xen/arch/x86/xen.lds.S                   |   1 +
 xen/common/Kconfig                       |   2 +
 xen/common/Makefile                      |   1 +
 xen/common/domain.c                      |   8 +-
 xen/common/emul/Kconfig                  |   6 +
 xen/common/emul/Makefile                 |   1 +
 xen/common/emul/vuart/Kconfig            |  24 +
 xen/common/emul/vuart/Makefile           |   2 +
 xen/common/emul/vuart/ns16x50.c          | 976 +++++++++++++++++++++++
 xen/common/emul/vuart/vuart.c            | 156 ++++
 xen/common/keyhandler.c                  |   3 +
 xen/drivers/char/console.c               |   6 +-
 xen/drivers/char/ns16550.c               |  16 +-
 xen/drivers/passthrough/x86/hvm.c        |  13 +-
 xen/include/xen/8250-uart.h              |  50 +-
 xen/include/xen/sched.h                  |   4 +
 xen/include/xen/serial.h                 |   3 +
 xen/include/xen/vuart.h                  | 116 +++
 xen/include/xen/xen.lds.h                |  10 +
 40 files changed, 1624 insertions(+), 183 deletions(-)
 create mode 100644 xen/arch/x86/ioport.c
 create mode 100644 xen/common/emul/Kconfig
 create mode 100644 xen/common/emul/Makefile
 create mode 100644 xen/common/emul/vuart/Kconfig
 create mode 100644 xen/common/emul/vuart/Makefile
 create mode 100644 xen/common/emul/vuart/ns16x50.c
 create mode 100644 xen/common/emul/vuart/vuart.c
 create mode 100644 xen/include/xen/vuart.h

-- 
2.51.0